Create `ec tag` to create a tag and push to the services repo

[GDYendell]

• Take the current year YY and month MM to create a prefix YY.MM
• Check the existing releases to add a number N for which release number in the month is next - or 1 if there are none.
• If the provided (or currently checked out) branch is not main, then it should add bX
• Prompt the user to confirm the new tag is correct
• Create tag and push
• Print created tag YY.MM.NbX

Subcommand name to be agreed. Could also be release.

Acceptance Criteria

• Can do ec tag and then ec deploy <service> <tag printed>