change: ETCM-12049 extrinsic to delete SPO metadata

[AmbientTea]

Description

• Adds logic to save depositor when creating SPO metadata
• Changes the signed message to make the metadata field optional, so it can be reused for deletion
• Adds a delete_metadata extrinsic to allow SPOs to clear their metadata and reclaim deposit
• Splits the metadata signing CLI command into upsert and delete subcommands; adds logic to sign deletion message

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

[LGLO]

I agree with Tomasz, that it will be simpler code and easier to understand when everything is in one map, but difference is so insignificant that I don't want to restrict PRs author freedom.