[feat] Add option to prefetch completion metadata and to persist completion metadata when switching schemas

[scottnemes]

Description

Currently completion suggestions only populate / apply for the selected schema, and when you switch schemas, all completions are cleared. This PR does a few important things:

1. Adds an option to prefetch completions for schemas automatically; this can be a list of schemas, or all schemas (or none, the default).
2. Prefetching of completions is done in the background when the client is launched.
3. When the user manually changes schemas (use command), completions are fetched for that schemas as usual. However if the user switches schemas again, the completions for the previously selected schema are now saved (this is independent of prefetching). So if you "use db1", then "use db2", completions for both db1 and db2 will now be available in addition to any prefetched schemas.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[rolandwalker]

Why off by default? Also, all is a valid schema name.