Handle candidate: prefix with UnmarshalCandidate

[JoeTurki]

Description

Make UnmarshalCandidate able to handle candidate: prefix in the candidate string.

Before I made UnmarshalCandidate strict, if a user passed a candidate from WebRTC candidates' toJSON, UnmarshalCandidate would incorrectly interpret the candidate: prefix as part of the foundation as showed here, leading to an incorrect and invalid candidate. Now that we enforce strict parsing, we correctly return an error in such cases. However, this change also introduces the ability to parse candidates with the candidate: prefix directly without any breaking changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.83%. Comparing base (37fb5d2) to head (ef453b3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #769   +/-   ##
=======================================
  Coverage   78.82%   78.83%           
=======================================
  Files          41       41           
  Lines        5435     5437    +2     
=======================================
+ Hits         4284     4286    +2     
  Misses        921      921           
  Partials      230      230           

Flag	Coverage Δ
go	78.83% <100.00%> (+<0.01%)	⬆️
wasm	27.10% <100.00%> (+0.02%)	⬆️

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