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.

[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.