perf(browser): gzip feature flag requests when available (#4657)

* perf(browser): gzip feature flag requests when available

* refactor(browser): reuse the feature flag compression request type

* test(browser): decode gzip feature flag payloads

* test(browser): support legacy flags payloads in compat tests

* fix(browser): preserve base64 flags fallback

* docs(browser): update compression fallback reference