Implement OpenID Connect relying-party support

[zgramana]

As an application architect, I want Sync Gateway to be able to use my existing SSO infrastructure so that I only need to maintain a single database of users identities and the resources that they have authorization to access, reducing both time-to-market and administrative overhead.

[adamcfraser]

[adamcfraser]

[adamcfraser]

Design notes: OpenID Connect Notes

[adamcfraser]

Subtasks:

• Internal OIDC OP testing endpoints #1772 - Internal Test OP
• Implement OIDC authentication endpoints #1773 - Auth flow endpoints
• Functional tests for OIDC authentication #1774 - OIDC functional testing
• Support SG authentication using OIDC ID token #1775 - SG authentication using JWT token