feat(provider): expose request metadata and header helpers

[decofe]

Motivation

Typed Provider methods return ProviderCall, which did not expose the request metadata mapping available on the underlying RpcCall. Callers that need request-scoped HTTP headers therefore had to drop down to the raw RPC client or manually unwrap the inner call.

Solution

• Add ProviderCall::map_meta, forwarding metadata changes to the underlying RpcCall.
• Add fluent with_headers and with_header helpers for request-scoped HTTP headers.
• Return the original call when metadata cannot be changed, matching the fallible map_resp API.
• Document per-request HTTP header usage and cover RPC and non-RPC variants.

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

Prompted by: @onbjerg