pjsip: add tests for multirate 2833

[mbradeen]

Demonstrate appropriate handling of non 8K RFC 2833 digits

Proposed addition of four categories of tests

1. Test that Asterisk offers the correct sdp for opus and ulaw when RFC 2833 is enabled
2. Test that Asterisk creates the correct responses for opus and ulaw when RFC 2833 is enabled
3. Test that Asterisk is able to read RFC 2833 digits embedded in both an opus stream and an ulaw stream.
4. Test that Asterisk generates the correct payload type for the dtmf digits in the rtp stream

Requires: #700

[github-actions[bot]]

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[mbradeen]

cherry-pick-to: 18 cherry-pick-to: 20 cherry-pick-to: 21

[github-actions[bot]]

Successfully merged to branch master and cherry-picked to ["18","20","21"]