Add telephone-event payload type

[dkirov-dev]

The "telephone-event" payload type is currently mapped to OTHER payload type.
This PR introduces TELEPHONE_EVENT payload type and maps "telephone-event" to TELEPHONE_EVENT.
This allows to have more accurate payload type map for PCAP capture functionality and debugging.

[dkirov-dev]

@bgrozev , @damencho Please review.

[JonathanLennox]

Jenkins, test this please

[JonathanLennox]

I'm not sure I quite understand the use case - how are you relating the payload types to the pcap capture?

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[dkirov-dev]

I'll create another draft PR for colibri initiated pcap capture where the use case will become clear, but I wanted to separate changes that can easily become part of the jitsi mainstream as being generic in nature vs changes that are less likely to make it.

[dkirov-dev]

@JonathanLennox please take a look at #2072 where the changes in this PR are used to generate a payload map such as

    {
      "payload_type": 126,
      "media_type": "audio",
      "clock_rate": 8000,
      "encoding": "telephone-event"
    },

instead of

    {
      "payload_type": 126,
      "media_type": "audio",
      "clock_rate": 8000,
      "encoding": "other"
    },

[dkirov-dev]

Essentially generating pcap files is not very useful unless there is metadata associated with the pcap file and describing the different payload types captured in the RTP packets.
Describing the payload map accurately requires correct mapping of the telephone-event and this is what this PR is achieving.

@JonathanLennox I hope this explains the rationale for this PR. Please review.