allow for stale branch cleanup (& dry run)

[mcncl]

Description

Enables the user to run a cleanup subcommand to delete branches which are left stale after a build has run to completion. Allows for --dry-run to be used.

Changes

• adds a cleanup command which will delete any branches with the pattern bk/preflight/*
  • works via using the [new] build branch resolver to get a list of branches with completed builds
• minor housekeeping with Kong structs (PreflightCmd -> RunCmd)

Testing

• Tests have run locally (with go test ./...)
• Code is formatted (with go fmt ./...)

[matthewborden]

I think we should allow providing a specific preflight UUID to target.

[mcncl]

@matthewborden would you want cleanup without args to clean up all branches, then if we supply --preflight-uuidwe only target that one? Or would you want UUID to be accepted as an optional positional arg, or allow the user to set --all?