feat: add sampleRate to PostHogSessionReplayConfig

[marandaneto]

💡 Motivation and Context

Add sampleRate config option to PostHogSessionReplayConfig to control session replay recording frequency. This mirrors the React Native implementation.

• Local SDK sampleRate takes precedence over remote config
• If sampleRate is not set (null), sampling is controlled by remote config via the native SDKs
• The value is forwarded to the native Android and iOS SDKs

Note: Native SDKs need sampleRate support on their PostHogSessionReplayConfig:

• Android: PR #437 (open)
• iOS: Planned (TODO in PostHogSessionReplayConfig.swift)

💚 How did you test it?

• All existing Flutter tests pass (93/93)
• Dart analyzer shows no new issues on changed files

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the release label to the PR