Adds endpoint to install extensions:
**POST** `/extensions`
```
{"extension":"pg_sessions_jwt","database":"neondb","version":"1.0.0"}
```
Will be used by `local-proxy`.
Example, for the JWT authentication to work the database needs to have
the pg_session_jwt extension and also to enable JWT to work in RLS
policies.
---------
Co-authored-by: Conrad Ludgate <conradludgate@gmail.com>
