rtp: not able to configure newly created header extension

According to the doc, the user code creating an extension as a reply to the request-extension signal is supposed to configure its attributes. But it's actually impossible for it to do it as it does not have access to the actual caps.

We have two options to fix this:

• pass the caps to the request-extension signal;
• call gst_rtp_header_extension_set_attributes_from_caps() in basepayload after the signal has been called.

I'd vote for the second option as it makes application code easier. We also already call it on caps update so it makes sense to have it there as well.

The depayload suffers the same issue.

cc @ystreet @ocrete