Creating example for new TokenProvider plugin

[stuart-wells]

TODO: Before merging, need to update go.mod to the actual release that TokenProvider is included with.

What was changed

Added samples for using JWT to auth replication/internode traffic

Why?

TokenProvider was added to OSS in temporalio/temporal@74a7108 and will soon be released. Prepping a sample for reference in setting up a server using the plugin.

Checklist

1. How was this tested:
   Used samples to create local servers and ran validation against those servers.

2. Any docs updates needed?
   Doc changes are already approved.

[Copilot]

Pull request overview

Adds a new extensibility sample demonstrating how to plug in Temporal Server’s auth.TokenProvider to attach bearer tokens to outbound cross-cluster (replication) RPCs, along with dependency updates needed to consume the (pre-release) Temporal Server changes that introduced TokenProvider.

Changes:

• Add a new extensibility/tokenprovider sample (provider implementation + standalone server wiring + tests + test config).
• Update the extensibility index README to link the new TokenProvider sample.
• Update extensibility/go.mod/go.sum to pin Temporal Server/API versions that include the new TokenProvider support, and adjust the authorizer claim-mapper TLS subject handling.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Adds a link to the new Token Provider sample.
extensibility/tokenprovider/README.md	Documents the TokenProvider sample, caching behavior, and required config knobs.
extensibility/tokenprovider/myTokenProvider.go	Implements a file-backed TokenProvider with expiry-aware caching via oauth2.ReuseTokenSource.
extensibility/tokenprovider/myTokenProvider_test.go	Unit tests for file token reading, caching behavior, and JWT exp parsing.
extensibility/tokenprovider/server/main.go	Wires temporal.WithTokenProvider(...) into a sample server.
extensibility/tokenprovider/server/main_test.go	Boots the sample server and validates it becomes reachable with TokenProvider required.
extensibility/tokenprovider/server/testdata/config.yaml	Minimal server config for tests, including remoteClusterAuth+TLS stub.
extensibility/go.mod	Pins Temporal dependencies (incl. TokenProvider commit) and updates module requirements.
extensibility/go.sum	Updates dependency checksums to match the new module graph.
extensibility/authorizer/myClaimMapper.go	Switches TLS-based claim extraction to use TLSSubject when present (mTLS).

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