allow setting org level billing related banners

[pjain1]

This allows setting org level warning/error banner using sudo cmds. They are only visible to org admins.

• Only a single message can be set, setting again overrides the previous message
• If the level is warning then orange banner is displayed, otherwise red.

rill sudo billing set-message --org <> --message "hello"  --level [warning|error]
...
rill sudo billing delete-message --org <>

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!