refactor(pg_lsp): use tokio, partially remove crossbeam channels

[juleswritescode]

This PR introduces tokio into the pg_lsp crate and tries to remove the crossbeam dependency from the LspServer.

• All separate tasks are now spawned with tokio
• All separate tasks listen for a cancel_token that allows us to shut down the server without dangling tasks
• The database connection now lives in its own file
• The ClientFlags now live in its own file
• I refactored the setup logic a bit, for example, the InitializeRequest flow now lives in its own function
• I split up the internal message loop into process_messages, handle_message and handle_internal_message
• Schema Updates & DbConnectionResets are now done via channels & internal messages
• and, last but not least, I added a tiny client.send_info_message helper. 🤓

Next steps:

• replace the lsp_types with tower_lsp, which should work better with tokio
• if possible, use tokio with both the LspClient and the EventDebouncer (haven't looked into that yet)
• migrate the lsp-server to an architecture that uses daemons, such as biome's

Looking forward to your feedback!

[juleswritescode]

the linter marked all of this as unused

[juleswritescode]

I didn't dare touch the EventDebouncer yet 🙈