OIDC authentication

[jackmcguire1]

Introduce Twitch OIDC authentication to Helix.

• Twitch user claims struct
• IDToken struct
• OIDC auth struct
• Request user OIDC access token
• UserInfo endpoint from access token

[coveralls]

Pull Request Test Coverage Report for Build 1186133813

• 0 of 67 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-5.5%) to 91.096%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
authentication.go	0	67	0.0%
<!--	Total:	0	67

Totals
Change from base Build 1179954764:	-5.5%
Covered Lines:	1064
Relevant Lines:	1168

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💛 - Coveralls