Full http in Rust support, remove httpx

[mmastrac]

Makes use of HttpClient throughout the entire server process:

• Auth
• AI extension (incl. SSE)
• std::net::http

Notes:

• httpx is no longer a dep (kept as a dev-dependency)
• Uses an EdgeDB ${version} User-Agent for all HTTP requests
• All HTTP requests are subject to http_max_connections config option. This option is refreshed from the std::net worker.
• SSE supports backpressure, and queue is fixed at 100 messages for now
• Streaming and non-streaming requests use separate APIs to simplify the implementation
• One thread per tenant is used if HTTP is required (we boot this thread only when needed). Eventually we'll move to a one-runtime-per-tenant and this thread will disappear.

[msullivan]

I decided to go get rid of httpx as a test dependency too: #7950