pdns provider: add an optional argument to set the server_id to a value instead of always localhost

[romeroalx]

What would you like to be added:
An optional argument to set the server_id to a value instead of always to localhost as it is currently set.

Why is this needed:
It is defined in the official documentation of the PDNS API that the value of the server_id should be almost always "localhost" except for the cases where there is a proxy in front of PDNS servers: See https://doc.powerdns.com/authoritative/http-api/server.html#servers