* Move tenant.Service unit tests into its package
* Delete the top-level TenantService interface now that it's not used.
* Move helper funcs for setting up test stores into testing pkg
* Delete duplicate implementations scattered through the codebase
* Move error assertions into store-creation helpers
This includes removal of a lot of kv.Service responsibilities. However,
it does not finish the re-wiring. It removes documents, telegrafs,
notification rules + endpoints, checks, orgs, users, buckets, passwords,
urms, labels and authorizations. There are some oustanding pieces that
are needed to get kv service compiling (dashboard service urm
dependency). Then all the call sites for kv service need updating and
the new implementations of telegraf and notification rules + endpoints
needed installing (along with any necessary migrations).
* refactor: migrator and introduce Store.(Create|Delete)Bucket
feat: kvmigration internal utility to create / managing kv store migrations
fix: ensure migrations applied in all test cases
* chore: update kv and migration documentation