Option to specify explicit entity-tag in PUBLISH request (thanks Johan Lantz for the suggestion)

[bennylp]

Dangling event publication sessions may occur in the server due to conditions such as network loss, access point change, and so on. In this case, it will be useful if application can specify an explicit etag to be tried by PJSUA-LIB.

If there is no such session in the server, the server will respond with 412 Conditional Request Failed response, and PJSUA-LIB should retry the publication without any etag/SIP-If-Match header. Otherwise the publication session will be updated with the new presence data.