[FFL-2929] Separate Flags implementation ownership

[leoromanovsky]

Motivation

FFE owns the Flags implementation. The current rules also require mobile review for internal changes that do not change the SDK public API.

Changes and Decisions

• Assign the Flags and Flags OpenFeature implementation paths to FFE.
• Keep mobile and FFE ownership on the generated API snapshot paths.
• Preserve the existing effective ownership of both Flags README files.
• Require mobile review when a Flags public API change updates an API snapshot.
• Keep Session Replay unchanged because the mobile team owns its implementation and public API.

[datadog-datadog-prod-us1]

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🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 71.76% (+0.02%)

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