Add federation whitelist for servers

[enaix]

Description

This PR introduces a mechanism to exclude all servers from federation except given ones. Similar functionality is implemented in Synapse using federation_domain_whitelist. This functionality has been requested by users: #3487 (comment)

Configuration

In federation:

• enable_whitelist: enable or disable whitelist
• whitelisted_servers: the list of server names to whitelist

Changes

• Added new table with the list of whitelisted servers
• Added necessary APIs for accessing this server property
• The whitelist check is performed in federationapi/internal/federationclient.go

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• Pull request includes a sign off below or I have already signed off privately

Sign-off: private

[CLAassistant]

All committers have signed the CLA.

[S7evinK]

Just a few quick notes:

• I'd prefer if we could use AllowList or similar instead of WhiteList
• No need for an Enable option, if we can just check len(allowedServers) > 0
• Do we actually need a database table, isn't the list static?

[enaix]

• I wanted to match the naming of synapse config options to avoid confusion, but sure
• Sure
• Yeah, it makes sense, since we can use a hashmap and drastically reduce cpu cycles

I wanted to ask if it's enough to add allowlist checks in federationclient.go

[enaix]

I wanted to ask if it's enough to add allowlist checks in federationclient.go

It seems that it's not enough.. clientapi/routing is performing various requests like public rooms fetch. Should we add whitelist checks to clientapi/routing/routing.go?