realtime: Increased reg_server character size

Currently, the ps_contacts table's reg_server column in realtime database type is varchar(20).
This is fine for normal cases, but if the hostname is longer than 20, it returns error and then
failed to register the contact address of the peer.

Normally, 20 characters limitation for the hostname is fine, but with the cloud env.
So, increased the size to 255.

ASTERISK-29056

Change-Id: Iac52c8c35030303cfa551bb39f410b33bffc507d