basertpendpoint: add externalIPv4 and externalIPv6 parameters

[dpocock]

What is the current behavior you want to change?

What is the new behavior provided by this change?

RTP endpoint currently selects the media IP address in a random manner.

On a multi-homed host this can be problematic.

How has this been tested?

Testing with my reConServer / reSIProcate / Kurento integration branch

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature / enhancement (non-breaking change which improves the project)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] My change requires a change to the documentation
• [x] My change requires a change in other repository kms-elements pull request submitted

Checklist

• [x] I have read the Contribution Guidelines
• [x] I have added an explanation of what the changes do and why they should be included
• [ ] I have written new tests for the changes, as applicable, and have successfully run them locally

[dpocock]

related change in kms-elements: https://github.com/Kurento/kms-elements/pull/36