Add searchStarterPacks to appview

[rafaelbsky]

This adds:

1. The lexicon app.bsky.graph.searchStarterPacks and its implementation in the bsky package.
2. The lexicon app.bsky.unspecced.searchStarterPacksSkeleton, to be implemented by the search service.

[rafaelbsky]

Not sure what to do with the term here, since we don't have the starter pack name stored in the appview. I think the search service will have it though, so that's why it exists.

I'm happy to change it to something better.

[devinivy]

We certainly could store the starter pack name in the db! How is it handled for post and actor search? Ideally we'd follow a similar pattern to those two if there is a pattern.

[rafaelbsky]

I added a search by term with a gin index. I'm doing ilike, not sure if it is a good usage of the index or how much we care about it.

[devinivy]

Looking good! The only thing we might want to consider here is indexing the starter pack names so that some form of search is implemented in the dataplane.

[devinivy]

P.S. let's add a changeset for the @atproto/api package to this PR, so that a version of that package containing the new search lexicon can be published.