Refactor ACRRequest

[Vladimir Dzhuvinov]

• Add own requirement enum to signify a mandatory, essential and voluntary request.
• Add a static resolve method that takes in OIDC client metadata, OP metadata, acr_values and claims as parameter and returns the matching ACR request.

[Vladimir Dzhuvinov]

Spec issue resolved June 27 2013 (draft 20):

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acr_values OPTIONAL. Requested Authentication Context Class Reference values. Space-separated string that specifies the acr values that the Authorization Server is being requested to use for processing this authentication request, with the values appearing in order of preference. The Authentication Context Class satisfied by the authentication performed is returned as the acr Claim Value, as specified in Section 2.1.2.1. The acr Claim is requested as a Voluntary Claim by this parameter.

[Vladimir Dzhuvinov]

Done in commit d2e6d72.