Fix type of CandidatePairState constants

[stv0g]

Fixes #571

@Sean-Der This is another unfortunate instance of an untyped enum. What do you think about fixing this instance as well?

It will break the API, however just like already had to do for the ConnectionState enum.

See also: pion/webrtc#2471 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.68%. Comparing base (8680cd5) to head (4acdb2b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #572   +/-   ##
=======================================
  Coverage   78.68%   78.68%           
=======================================
  Files          42       42           
  Lines        4466     4466           
=======================================
  Hits         3514     3514           
  Misses        734      734           
  Partials      218      218           

Flag	Coverage Δ
go	78.68% <ø> (ø)
wasm	23.50% <ø> (ø)

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