Add cloud backup agent

[ludeeus]

Breaking change

Proposed change

Relies on:

• MVP implementation of Backup sync agents #126122
• Add WS handler to generate encryption key for cloud backup #129771
• Add cloud preference for backup sync #130355

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

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[home-assistant]

Hey there @home-assistant/cloud, mind taking a look at this pull request as it has been labeled with an integration (cloud) you are listed as a code owner for? Thanks!

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[MartinHjelmare]

We should rebase this to allthebackupchanges branch.