Implement token caching with automatic renewal

[Noahnc]

Replace the current AuthenticationInterceptor with a wrapped HttpHandler that fetches tokens on-demand during requests.

This also fixes Issue 283 where the watch can no longer re-establish once the cached token has expired.

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[Copilot]

Pull request overview

This PR reworks how EtcdClient applies authentication by moving from a gRPC Interceptor that reads a cached token to an HttpMessageHandler that fetches and caches tokens on-demand, enabling automatic renewal during reconnects (notably for watch streams).

Changes:

• Replace AuthenticationInterceptor token injection with a new AuthenticationHttpHandler that fetches/caches tokens and bypasses the Authenticate RPC to avoid recursion.
• Update EtcdClient authentication APIs to support token cache duration configuration and credential clearing/invalidation.
• Replace unit tests for the removed interceptor with new unit tests for the handler; adjust integration tests and update auth docs.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
dotnet-etcd/etcdClient.cs	Installs AuthenticationHttpHandler, adds credential/token-cache configuration and lazy token acquisition via RequestTokenAsync.
dotnet-etcd/AuthenticationInterceptor.cs	Removes the old interceptor-based auth approach.
dotnet-etcd/AuthenticationHttpHandler.cs	Introduces a delegating handler to attach, cache, and refresh auth tokens on outgoing requests.
dotnet-etcd.Tests/Unit/AuthenticationInterceptorTests.cs	Removes tests for the deleted interceptor.
dotnet-etcd.Tests/Unit/AuthenticationHttpHandlerTests.cs	Adds unit coverage for header injection, caching, invalidation, and concurrency behavior in the new handler.
dotnet-etcd.Tests/Integration/AuthClientIntegrationTests.cs	Minor updates and ensures credentials can be cleared during auth setup/teardown flows.
docs/authentication/api-reference.md	Documents the new token cache duration parameter and updated credential behavior.

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[shubhamranjan]

Thank you for this feature. Please look into the failing tests. (Try rebasing first)

[Noahnc]

@shubhamranjan The failing integration test seems to be unrelated to my change and was fixed in PR-293. I just merged the fix into this branch.
I also addressed the issues found by Copilot.

[shubhamranjan]

cool, thank you. I will test this branch myself locally to see if all is fine.