feat: support custom sourcemap intake endpoint for private deployments

[Fiona2016]

What

Adds a sourcemapEndpoint option to the plugin so ProGuard/R8 mapping files can be uploaded to a self-hosted RUM ingest (private deployment) instead of only the predefined Flashcat sites (CN / STAGING).

The endpoint can be provided three ways, resolved with the precedence extension DSL > flashcat-ci.json > FLASHCAT_SOURCEMAP_INTAKE_URL env var. If the value omits the /sourcemap/upload path it is appended automatically (trailing slashes / surrounding whitespace are tolerated).

flashcat {
    sourcemapEndpoint = "https://rum.example.com" // -> https://rum.example.com/sourcemap/upload
}

Why these two extra fixes

While verifying the feature two issues surfaced:

1. Inconsistent precedence. sourcemapEndpoint resolved env before the CI config file (env wins), whereas site resolves CI config before env (CI wins). A user setting both an env var and flashcat-ci.json would get opposite behavior for the two settings. Aligned sourcemapEndpoint to the existing site order: extension > CI config > env.

2. API-key validation / error pointed at the public site host. On an upload error the plugin validated the API key against https://api.<site>/api/v1/validate and reported site=<public host> — wrong for a private deployment on a different host. Now, when a custom endpoint is in use, the validation call is skipped (the validate URL for a private deployment is unknown) and errors reference the actual upload endpoint.

Tests

• OkHttpUploaderTest (24): custom-endpoint upload, path resolution (resolveSourcemapUploadEndpoint), plus new cases for custom-endpoint 403 (→ InvalidApiKeyException, no validation request) and 400 (validation skipped).
• MappingFileUploadTaskTest (26): three resolution sources + precedence; flipped the env-vs-CI test to assert CI now wins.
• DdAndroidGradlePluginTest (34): config wiring.

All 84 tests pass locally (failures=0, errors=0).

Note: ./gradlew :…:build fails locally only because the bundled JaCoCo agent can't instrument JDK 21 (Unsupported class file major version 65). CI runs JDK 17, so this does not affect the pipeline.

Docs

README.md and 快速开始.md updated with the new option, env var, and flashcat-ci.json key.

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