snapshot: add 'snapshot summary', 'snapshot test'

[bobheadxi]

These commands are tooling intended for use in on-prem-to-Cloud data migrations:

• src snapshot summary will generate a summary snapshot of an instance. APIs used here must be available in Sourcegraph 3.20 and later - see RFC 760
• src snapshot test will use the summary and other health indicators to try and show that the instance is healthy and in an expected state post-migration. APIs used here can be in the latest releases of Sourcegraph, because we will run a multi-version upgrade in Cloud to the latest version.
  • The instance health checking components of this can in the future be re-used in a different command

Closes https://github.com/sourcegraph/customer/issues/1612 , closes https://github.com/sourcegraph/customer/issues/1619

Also part of this tooling eventually will be other user-facing utilities like:

1. database dumping for common deployment setups
2. database dump and summary upload utilities to Sourcegraph-controlled GCS buckets

See https://github.com/sourcegraph/customer/issues/1525 for more details

Test plan

plus a wee bit of unit tests.