[WIP] feat: Async refactor

[milutinke]

Async refactor

Migrating the client to use proper async/await throughout, following Microsoft's guidelines and common .NET practice. The groundwork was laid in #2385; this PR picks up that thread.

This is heavy work and will land in multiple stages.

It also will pull in the faster AES and stream improvements from #2385 in next stages.

Once this is merged and stable, the next plans are:

• Adding missing packet handlers
• Updating handlers that currently drop data to actually parse and store it
• Adding records for packet data where they're missing
• A codec-based approach to parsing packets, entity metadata, enchantments, particles, and similar structures across protocol versions

After which we can start creating a separate library once we have all of these completed and working reliably.