Backup owner update (for multi-service refactor branch)

[jmoreira-valory]

Proposed changes

Backup owner update (for multi-service refactor branch).

Note:
After some discussions with @truemiller and @iasonrovis, it was decided to simplify the endpoint, so that the client must provide explicitly the chain and (optionally) the backup owner. No multiple chains can be specified in a single call.

The reason for that is that:

• This will allow the frontend to monitor progress when several chains need to be changed (if we make all changes in a single call, the middleware can't provide progress information).
• It allows to fine-tune the process of selecting which chains need to be updated. For example, if a GUI where the user ticks the boxes of the chains they want to update.
• It avoids the problem of sending multiple chain updates with the same backup owner address where this may create conflicts (for example, we can't set the same backup owner for EVM and Solana chains).

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)