cache plan name

[pjain1]

Fixes https://github.com/rilldata/rill-private-issues/issues/973

• Cache and update plan name on performing any subscription related action
• Cached plan name is returned as part of GetOrganization rpc
• GetBillingSubscription does not use cache always fetches afresh so should not be used on each page load but only on billing page
• For handling external changes (directly done in Orb, although it should not be done), webhook are used to update the cache. They can be removed if we are ok with some staleness when external modifications are done.

[begelundmuller]

Since this is updated with webhooks, it should be consistent, right? So can it be stored in Postgres instead?

It would be useful to use Postgres because a) we have multiple replicas of the admin service, b) it makes introspection easier and provides easy access to the plan name in downstream pipelines