Generic JSON Caching

[ChrisPenner]

• Create new table on staging and prod
• Merge Adds FromJSON instances to enable Share Persistent JSON caching. unisonweb/unison#5576

Overview

Redis caching is fast, but is ephemeral, there are some things where it makes sense to keep longer-lived caches, especially since using immutable hashes means we often don't ever need to bust the cache.

This adds a generic mechanism for long-lived caches which are backed by Postgres.

Implementation notes

• Adds a new table which stores json blobs keyed by codebase, topic, and an arbitrary cache key.
• Adds usingJSONCache helper for easily creating/using cached values.

Test coverage

Tested locally.