New configuration to append custom headers into any requests generated by a particular account (thanks Johan Lantz for the suggestion)

[bennylp]

We already have the facility to insert custom header(s) into outgoing requests via pjsua_msg_data facility, but this lacks the capability to add the header into REGISTER request.

This ticket adds new fields in pjsua_acc_config to allow application to specify list of headers to be appended to particular requests generated on behalf of the account. A bitmask field may be used to control which headers may go into which requests.