res_pjsip_header_funcs: Add new PJSIP_INHERITABLE_HEADER dialplan function

res_pjsip_header_funcs: Add new PJSIP_INHERITABLE_HEADER dialplan function

Adds a new PJSIP_INHERITABLE_HEADER dialplan function to add
inheritable headers from the inbound channel to an outbound
bridged channel.  This works similarly to the existing
PJSIP_HEADER function, but will set the header on the bridged
outbound channel's INVITE upon Dial.

Inheritable headers can be updated or removed from the inbound
channel as well as from a pre-dial handler

Resolves: #1670

UserNote: A new PJSIP_HEADER option has been added that allows
inheriting pjsip headers from the inbound to the outbound bridged
channel.
Example- same => n,Set(PJSIP_INHERITABLE_HEADER(add,X-custom-1)=alpha)
will add X-custom-1: alpha to the outbound pjsip channel INVITE
upon Dial.