Support read_action fallback for list-only properties (GH-1167)

[jaredfholgate]

What

Adds optional read_action, read_action_method, and read_action_response_path arguments to azapi_resource and azapi_update_resource, so property values can be read from a list-style action when a plain GET returns them empty.

Why

Fixes #1167. A GET on Microsoft.Web/sites/config/appsettings returns properties: {} — the real values are only exposed via POST .../config/appsettings/list. azapi's read-merge is config-body-driven, so an empty GET drops every configured key and reports perpetual drift (notably for Flex Consumption function apps).

Behaviour

• Default (unset) — automatic: if the GET covers none of the configured body leaves and a list function exists in the embedded schema for the type + api-version, fall back to POST {resource_id}/list and merge that response. After the POST it re-checks overlap and warns + falls back to the GET response if the POST also covers nothing.
• "none" — force the current GET-only behaviour.
• any other value (e.g. "list") — force POST {resource_id}/{value}. read_action_method overrides the verb; read_action_response_path (JMESPath) lifts a sub-object before the merge.

The merge stays config-body-driven, so a mismatched response is a harmless no-op.

This also fixes a value-vs-pointer type assertion in LoadFunctionTypeDefinition that prevented resource function definitions from ever resolving.

Testing

• go build ./..., go vet, gofmt -s: clean
• golangci-lint run (v2.11.4): 0 issues
• New unit tests: Test_countCoveredLeaves, Test_applyResponsePath
• New acceptance tests exercising the Continuous drift updating flex consumption app config #1167 scenario (Flex Consumption Microsoft.Web/sites/config app settings — both the automatic fallback and an explicit read_action = "list"):

--- PASS: TestAccGenericUpdateResource_readActionAppSettings (95.96s)
--- PASS: TestAccGenericUpdateResource_readActionAppSettingsExplicitList (99.37s)
PASS
ok  	github.com/Azure/terraform-provider-azapi/internal/services	112.457s

Changelog & docs

• CHANGELOG.md: entry under v2.11.0 ENHANCEMENTS.
• docs/resources/resource.md and docs/resources/update_resource.md updated for the new arguments.