Support for alphanumeric numbers

[chancla784797]

I could not find anything related to this feature, how to send SMS using an alphanumeric number (i.e. YateBTS instead of 12345) ?

[chancla784797]

https://github.com/yatevoip/yatebts/blob/9b289b413c93e882bc4a0722060e94f5538ba1aa/ybts.cpp#L2403C49-L2403C49
Looks like to be the issue. It is currently possible to encode only unknown, international and national types

[chancla784797]

@monicatepelus