RNN version update on Engine automation

[markdevocht]

Related to https://github.com/wix-private/mobile-apps-engine/pull/3192

Description

RNN version update script that runs via the following flow:

RNN side — .github/workflows/update-engine.yml:

• Triggers on release publish (+ manual dispatch for testing)
• Checks for wix-labeled PRs merged since the previous release
• If found, sends a repository_dispatch to wix-private/mobile-apps-engine with the version, previous version, and PR list

Engine side — .github/workflows/update-rnn.yml:

• Receives the rnn-version-update dispatch
• Polls npm view react-native-navigation@ against the Wix registry every 5 minutes, up to 2 hours
• Once available: bumps both package.json files, runs yarn install --mode update-lockfile
• Creates a PR via peter-evans/create-pull-request (matching engine's existing pattern) on branch automated/rnn-update- with the Upgrade label

TODO:
Secret needed: ENGINE_GITHUB_TOKEN in the RNN repo with repo scope on wix-private.