feat: Neon-compatible HTTP /sql endpoint

Adds POST /sql HTTP endpoint compatible with the HTTP mode of the @neondatabase/serverless driver.

It follows a similar architecture to SupavisorWeb.WsProxy, where each request is handled as a regular Supavisor client. We first parse, understand, and build the queries (from Neon requests) to send to the DB in Supavisor.HttpSql, and then Supavisor.HttpSql.Client, which acts as a local client to Supavisor, runs them over a local connection to the transaction port.

Closes #152