feat(realtime): add metadata to realtime user broadcast push

[edgurgel]

🔍 Description

In the future we want to be able to include some metadata using JSON to User Broadcast Push.

What changed?

Add an optional metadata to be sent as part of the User Broadcast Push. At the moment it won't send anything because we don't have any allowed metadata keys but the serializer is ready to accept those as an option.

We don't have any usage at the moment for this extra metadata but we want to have this ready for the future

Related to supabase/realtime#1629

📸 Screenshots/Examples

🔄 Breaking changes

• This PR contains no breaking changes

No breaking changes because 2.0.0 has not been set as default serializer and not publicly advertised just yet.

📋 Checklist

• I have read the Contributing Guidelines
• My PR title follows the conventional commit format: <type>(<scope>): <description>
• I have run npx nx format to ensure consistent code formatting
• I have added tests for new functionality (if applicable)
• I have updated documentation (if applicable)

📝 Additional notes

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commit: 1c7d8c5

[coveralls]

coverage: 95.367% (+14.2%) from 81.184%
when pulling 1c7d8c5 on feat/realtime-user-broadcast-push-metadata
into 2ab882b on master.