feat: add Apache HttpClient 5.x adapter support

[uuuyuqi]

Describe what this PR does / why we need it

Add a new sentinel-apache-httpclient5-adapter module to support flow control for outgoing HTTP requests made with Apache HttpClient 5.x.

Apache HttpClient 5.x has been the recommended version since 2020, and 4.x is now in maintenance mode. This PR adds first-class support for the 5.x API.

Does this pull request fix one issue?

Fixes #3612

Describe how you did it

Module structure (sentinel-adapter/sentinel-apache-httpclient5-adapter):

• SentinelApacheHttpClient5Handler — Implements ExecChainHandler to intercept outgoing requests in the HttpClient 5.x exec chain. Uses SphU.entry() for Sentinel resource entry with EntryType.OUT.
• SentinelApacheHttpClientConfig — Configuration class with pluggable prefix, resource extractor, and fallback.
• ApacheHttpClientResourceExtractor / DefaultApacheHttpClientResourceExtractor — Resource name extraction. Default format: METHOD:url with query string and fragment stripped (consistent with the existing OkHttp adapter).
• ApacheHttpClientFallback / DefaultApacheHttpClientFallback — Fallback on block. Default throws SentinelRpcException.

Key design decisions:

1. Uses ExecChainHandler interface — the idiomatic HttpClient 5.x interception mechanism, replacing the decorateMainExec approach used in the 4.x adapter.
2. Resource name format METHOD:url — consistent with the existing sentinel-okhttp-adapter convention, and auto-strips query/fragment for stable resource keys.
3. Empty resource name bypass — if extractor returns null/empty, Sentinel protection is skipped.
4. Minimum HttpClient version 5.1 (provided scope).

Describe how to verify it

1. Build: mvn compile -pl sentinel-adapter/sentinel-apache-httpclient5-adapter -am
2. Test: mvn test -pl sentinel-adapter/sentinel-apache-httpclient5-adapter
3. All 21 tests pass (7 unit tests for extractor/config/fallback + 5 integration tests with Spring Boot + 9 from extractor combinations).

Usage example:

CloseableHttpClient httpclient = HttpClients.custom()
    .addExecInterceptorBefore(ChainElement.MAIN_TRANSPORT.name(), "sentinel",
        new SentinelApacheHttpClient5Handler())
    .build();

Special notes for reviews

• The extension point interfaces (ApacheHttpClientResourceExtractor, ApacheHttpClientFallback) follow the same pattern as the existing 4.x adapter and OkHttp adapter.
• The DefaultApacheHttpClientResourceExtractor uses request.getUri().toString() which returns the full URL in the exec chain context. Query parameters and fragments are stripped for stable resource naming.
• HttpClient 5.x dependency is provided scope — users bring their own version (5.1+).

Made with Cursor

[uuuyuqi]

Closing this PR as it was incorrectly based on master (2.0.0-alpha2-SNAPSHOT). Re-submitting against the bump-version-1.8.10-snapshot branch.