Validate issuer when retrieving discovery document

[JuliusPC]

From the OIDC Discovery spec, section 4.3. OpenID Provider Configuration Validation:

If any of the validation procedures defined in this specification fail, any operations requiring the information that failed to correctly validate MUST be aborted and the information that failed to validate MUST NOT be used.

The issuer value returned MUST be identical to the Issuer URL that was directly used to retrieve the configuration information. This MUST also be identical to the iss Claim value in ID Tokens issued from this Issuer.

This library does not validate this in getWellKnownConfigValue().