[FR] Expose all symbols that refer to SRT protocol details to a separate public header file

[ethouris]

For applications that might require to operate on the SRT protocol directly, which has also an open definition, it would be useful if all symbols that refer to anything related to the SRT protocol is provided in a public header file, that is, for example:

• UMSG_* symbols that refer to control message type
• ACKD_* symbols that refer to ACK structure fields
• symbols that refer to bit layout in the SRT header (together with bits utilities)