FreeSWITCH-FS-7269

ss7: add ss7_iam_nature_connection_hex to support Nature of Connection
     Indicator in IAM message.
     - ss7_iam_nature_connection_hex is the channel variable to carry
       HEX value from/to IAM message
     - X-FreeTDM-IAM-NATURE-CONN-HEX is the x-header string to carry
       HEX value from/to sip invite message
     - This IE is 8 bit hex, which turns into 2 chars
     - Only Satellite indicator, Continuity check indicator, and Echo
       control device indicator are supported. Spare value are not taken.
       Spare value is always set to all zero.
     - example:
       To set 00010101 (bit H-A), set X-FreeTDM-IAM-NATURE-CONN-HEX string
       value to 15 in the sip invite message to NSG.
       When NSG send out sip invite, if the incoming IAM message comes with
       Nature of Connection IE, NSG will put X-FreeTDM-IAM-NATURE-CONN-HEX
       in the invite x-header. For example, if the IE value is 00001010 in
       the IAM, NSG will send X-FreeTDM-IAM-NATURE-CONN-HEX with value string
       of "0A".
     - Two characters are madatory. "0A" must be specified rather than "A". If
       invalid format or characters are found, this x-header will be ommited.
     - Valid characters include 0-9, A-F, and a-f