Set access token claim on HTTP request context

[stephen-soltesz]

This changes how the token controller validates access tokens.

1. The token issuer is configured via a flag.

2. Claims are validated without an explicit "Subject".

   • The claim subject will be used differently for various use-cases, such as for monitoring, a specific IP address in the envelope service, or target service like "ndt".

3. Claims are saved in the HTTP request context value using SetClaim() and can be retrieved using GetClaim().

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 17

• 20 of 20 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 99.107%

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Totals
Change from base Build 13:	0.03%
Covered Lines:	222
Relevant Lines:	224

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💛 - Coveralls

[yachang]

Reviewable status: 0 of 1 approvals obtained (waiting on @yachang)

[yachang]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @yachang)