Add Support for Parsing Extensions in Candidates

[JoeTurki]

The UnmarshalCandidate function should be improved to align more y with the ICE Candidate parsing rules outlined in RFC 5245, Section 15.1.

In addition, candidateBase should support access to extension values. This includes:

Parsing and storing extensions in a structured way.
Matching specific extensions for validation or other purposes.
Serializing (marshaling) the candidate, including its extensions, back into a string.

This is required for many issues like pion/webrtc#2993

I'm making a PR.