Feat/feed

[Nuhvi]

For early review and testing with Pubky Nexus, this PR adds a feed of events (PUT and DELETE) for added or deleted files.

The API works as follows:

1. Call GET <homeserver base URL>/events/ (note the trailing /)
2. You should get a text/plain response, made out lines separated with \n char
3. Last line contains the cursor to be used for the next request. You should be able to parse it by getting the last 13 characters in the body.
4. Each other line starts with PUT or DEL followed by a space char then the pubky URL for the resource.
5. You can use limit and cursor query params, no reverse.

The assumption, is that you will keep making requests until you get a response with header content-length = zero. Then you back off, and make the next request after some interval, if you don't, we will eventually add a rate-limiting anyways.

Things left to discuss:

1. Should we return the cursor in an http header pubky-cursor instead of last line? I am trying to avoid JSON because I dislike it as a dependency, especially when it is not needed.
2. If your backoff/interval is too long, for example minutes, then we might need to add a subscription API so that the server notify you once it has something new before that interval passes, but ... no urgent need for that I thing.
3. What else?

This is a stopgap or at least an incomplete solutions, since it only works for realtime sync, but it fails in two situations:

1. If you start a new Indexer from scratch, you will have to list A LOT of events to sync with a homeserver, most of these events are redundant (PUT event(s) that is later negated by DEL event), and even worse, homeservers will be expected to delete events older than X amount of days so that makes syncing incomplete.
2. If a user migrates from one homeserver to another, you don't want the indexer to detect that as a bunch of new events, if it already seen all that information.

For both these cases, we simply have to use set-reconciliation, and for that, we need a Merkle tree of sorts.

[SHAcollision]

Can be merged as currently is! It is fully functional.

I will create issues as needs get discovered :)