Support snapshotting original downstream and upstream header context

[SavinduDimal]

Description

• Related Discussions: Expose Original Client Headers to Policies via Downstream/Upstream Context #2735
• This issue was created to track the progress of the fix for the bug reported in [Bug]: Set Header policy takes effect before mcp_auth policy #2548

[SavinduDimal]

Update

• During the initial discussions it was decided to go with the fix given in comment [1] with following new types in the SDK.

DownstreamContext

type DownstreamContext struct {
    Headers *Headers
}

UpstreamContext

type UpstreamContext struct {
	Headers *Headers
}

Changes to Context Structs

{
  "RequestHeaderContext": {
    "SharedContext": "(embedded)",           // existing
    "Headers":       "Headers",              // existing
    "Path":          "string",               // existing
    "Method":        "string",               // existing
    "Authority":     "string",               // existing
    "Scheme":        "string",               // existing
    "Vhost":         "string",               // existing
    "Downstream": {                          // NEW
      "Headers": "Headers"
    }
  },

  "RequestContext": {
    "SharedContext":  "(embedded)",          // existing
    "Headers":        "Headers",             // existing
    "Body":           "Body | null",         // existing
    "Path":           "string",              // existing
    "Method":         "string",              // existing
    "Authority":      "string",              // existing
    "Scheme":         "string",              // existing
    "Vhost":          "string",              // existing
    "Downstream": {                          // NEW
      "Headers": "Headers"
    }
  },

  "RequestStreamContext": {
    "SharedContext": "(embedded)",           // existing
    "Headers":       "Headers",              // existing
    "Path":          "string",               // existing
    "Method":        "string",               // existing
    "Authority":     "string",               // existing
    "Scheme":        "string",               // existing
    "Vhost":         "string",               // existing
    "Downstream": {                          // NEW
      "Headers": "Headers"
    }
  },

  "ResponseHeaderContext": {
    "SharedContext":   "(embedded)",         // existing
    "RequestHeaders":  "Headers",            // existing — headers forwarded to upstream
    "RequestBody":     "Body",               // existing
    "RequestPath":     "string",             // existing
    "RequestMethod":   "string",             // existing
    "ResponseHeaders": "Headers",            // existing
    "ResponseStatus":  "int",               // existing
    "Downstream": {                          // NEW
      "Headers": "Headers"
    },
    "Upstream": {                          // NEW
      "Headers": "Headers"
    }
  },

  "ResponseContext": {
    "SharedContext":   "(embedded)",         // existing
    "RequestHeaders":  "Headers",            // existing — headers forwarded to upstream
    "RequestBody":     "Body",               // existing
    "RequestPath":     "string",             // existing
    "RequestMethod":   "string",             // existing
    "ResponseHeaders": "Headers",            // existing
    "ResponseBody":    "Body | null",        // existing
    "ResponseStatus":  "int",               // existing
    "Downstream": {                          // NEW — original client request headers
      "Headers": "Headers"
    },
    "Upstream": {                          // NEW
      "Headers": "Headers"
    }
  },

  "ResponseStreamContext": {
    "SharedContext":   "(embedded)",         // existing
    "RequestHeaders":  "Headers",            // existing — headers forwarded to upstream
    "RequestBody":     "Body",               // existing
    "RequestPath":     "string",             // existing
    "RequestMethod":   "string",             // existing
    "ResponseHeaders": "Headers",            // existing
    "ResponseStatus":  "int",               // existing
    "Downstream": {                          // NEW — original client request headers
      "Headers": "Headers"
    },
    "Upstream": {                          // NEW
      "Headers": "Headers"
    }
  }
}

• Added above changes with PR: [SDK] Add Changes to Clone Downstream and Upstream Header Contexts #2631. And tested them with a custom policy to check the header context and previous live headers in both request and response phases (both header and body).
• Updated the existing policies which can be affected by the originally reported bug with new snapshot header contexts.
• However during the development process a new requirement came with the new field UpstreamInfo which needs to contain upstream URL and basepath. Hence the above introduced types and context changes will be replaced by the changes mentioned in the comment [1].

[1] - #2735 (comment)