Write `pkg/auth/spannerauth/spannerauthmigration` package

[amwolff]

Goal

For https://github.com/storj/storj-private/issues/440, we need to write a migration package for migration from badgerauth to spannerauth, similar to https://review.dev.storj.io/c/storj/gateway-mt/+/7542.

Acceptance Criteria

• The change introducing the migration package has a follow-up change with tests
• The migration backend writes to two destinations
  • and reads from spannerauth first, failing over to badgerauth
  • failover should be recognizable (metrics); they will be unexpected
• There's no need for global downtime (all auth services are down) when it's enabled
• Local downtime (one auth service at a time down) is perfectly fine

Links

• https://github.com/storj/storj-private/issues/440
• https://review.dev.storj.io/c/storj/gateway-mt/+/7542

[storj-gerrit[bot]]

Change pkg/auth: wire up spannerauth to the command line interface. mentions this issue.

[storj-gerrit[bot]]

Change pkg/auth/spannerauth/spannerauthmigration: add package mentions this issue.

[storj-gerrit[bot]]

Change pkg/auth/spannerauth/spannerauthmigration: add tests mentions this issue.

[storj-gerrit[bot]]

Change pkg/auth: wire up spannerauthmigration to command line interface mentions this issue.

[storj-gerrit[bot]]

Change pkg/auth/spannerauth: fix incorrect size of secret key column mentions this issue.

[storj-gerrit[bot]]

Change {cmd/authservice,pkg/auth}: add concurrency to spannerauthmigration mentions this issue.

[storj-gerrit[bot]]

Change pkg/auth: update logging for consistency mentions this issue.