Added caching to the LinkRedirectRepository

[cmraible]

ref https://linear.app/tryghost/issue/ENG-851/implement-a-minimal-but-complete-version-of-redirect-caching-to
ref https://app.incident.io/ghost/incidents/55

Often immediately after sending an email, sites receive a large volume of requests to redirect endpoints from members clicking on the links in the email.

We currently don't cache any of these requests in our CDN, because we also record click events, update the member's last_seen_at timestamp, and send webhooks in response to these clicks, so Ghost needs to handle each of these requests. This means that each of these redirect requests hits Ghost, and currently all these requests hit the database to lookup where to redirect the member to.

Each one of these requests can make up to 11 database queries, which can quickly exhaust Ghost's database connection pool. Even though the redirect lookup query is fairly cheap and quick, these queries aren't prioritized over the "record" queries Ghost needs to handle, so they can get stuck behind other queries in the queue and eventually timeout.

The result is that members are unable to actually reach the destination of the link they clicked on, instead receiving a 500 error in Ghost, or it can take a long time (60s+) for the redirect to happen.

This PR uses our existing adapterManager to cache the redirect lookups either in-memory or in Redis (if configured). This only removes 1 out of 11 queries per redirect request, so it won't reduce the load on the DB drastically, but it at least decouples the serving of the redirect from the DB so the member can be redirected even if the DB is under heavy load.

Local load testing results have shown a decrease in response times from 60 seconds to ~50ms for the redirect requests when handling 500 requests per second, and reduced the 500 error rate to 0.