Consider modifying OpenIdConnectAuthenticationOptions to use a single ISecurityTokenValidator

[brentschmaltz]

Currently OpenIdConnectAuthenticationOptions.SecurityTokenValidators is a collection of validators, given that the specification requires a JWT, a collection here is not necessary.

Suggest change from ICollection -> ISecurityTokenValidator.

[kevinchalet]

Covered by #276 (comment)

[Eilon]

Close when the PR goes in.