Epic: Support `dblink`, `http`, and `postgres_fdw` extensions

[vadim2404]

Motivation

dblink, http, and postgres_fdw extensions are quite popular in analytical, migration, and sharding use cases. Therefore we need to install them.

DoD

As a user, I can install these two extensions and use them.

Implementation ideas

Tasks

• [ ] Ensure that computes use JWT token while communicating with SKs and PSs
• [ ] Enable network policies that allow traffic only for selected pageserver, safekeepers, proxy, and internet traffic
• [ ] #4704
• [ ] Enable these extensions

Other related tasks and Epics

• neondatabase/cloud#5745

[vmatt]

Hey, when this can be expected?