[FFL-3355] Add standalone rules configuration fetch helper - #1508

What does this PR do?

Add fetchRulesConfiguration and RulesConfigurationFetchOptions to the React Native OpenFeature /rules-based entry point (FFL-3355).

• Fetch client-distributed protobuf rules with a client token, environment, supported site domain, optional fetch implementation, and optional abort signal.
• Decode a portable FlagsConfiguration and attach the retrieval timestamp without initializing the native SDK or registering tracking.
• Validate sites before sending the token and preserve useful HTTP errors, network errors, cancellation, and decoding failures.
• Make one request per call, without implicit polling, retries, persistence, or tracking.
• Add documentation and regression coverage for explicit refresh, preserved configuration after refresh failure, and refreshed tracking metadata with active hooks.

Motivation

The RN core provider supports local rules-based evaluation, but applications also need a client-side fetch helper to obtain its portable configuration. This follows the browser helper pattern while avoiding URL APIs that are incomplete on supported RN versions.

Additional Notes

This is stacked on #1507 and intentionally targets sameerank/FFL-3355/native-tracking-hooks. The PR diff contains only the fetch helper and its related documentation/tests. Retarget to develop after the tracking PR lands.

The helper itself does not require native SDK initialization. The active-hook integration tests use the tracking functionality from the base branch. Native dependency/release blockers for the combined branch are documented in the tracking PR.

Validation:

• The targeted fetch-helper and tracking integration suites passed together: 51 tests.
• Tests cover supported/invalid domains, environment encoding, headers, cancellation, HTTP/network/body/decode failures, local evaluation after context changes, and failed/successful refresh with active tracking hooks.
• Full repository CI remains to be reviewed on this draft.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change