Allow using auto_accept_invites for specific servers over federation (allowlist)

**Description:**

Currently the [auto_accept_invites](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#auto_accept_invites) works either for local users only or for **any** invite from **any** server over federation.
It would be spectacular if there was an optional allowlist where you could define the list of servers you'd like to automatically accept invites from