Feat: added new auto join channels functionality

[davidejensen]

What does this PR change?

Fix #3158 Adds the possibility of defining a feature flag in unleashed to create a list of channels to force users to auto join with the possibility of enabling/disabling the notifications by default

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How to test the changes?

1. Go to: https://play.decentraland.zone/?renderer-branch=feat/auto-channel&kernel-branch=fix/channels-messages
2. Verify that the automatically joined channels are the ones typed here: https://features.decentraland.systems/#/features/variants/explorer-automatic_joined_channels
3. Verify that the notifications for those channels are enabled and disabled based on the FF variant definition
4. More info at #3158

Our Code Review Standards

https://github.com/decentraland/unity-renderer/blob/master/docs/code-review-standards.md

[github-actions[bot]]

After the CI passes:

• This branch can be previewed at https://play.decentraland.zone/?renderer-branch=feat/auto-channel
• Code coverage report: https://renderer-artifacts.decentraland.org/branch-coverage/feat/auto-channel
• Benchmark report: https://renderer-artifacts.decentraland.org/branch-benchmark/feat/auto-channel/index.html

[davidejensen]

Closing in favour of https://github.com/decentraland/unity-renderer/pull/3351