Add classes for RequestedAuthnContext structure

[tvdijen]

Adds classes + unit tests for: RequestedAuthnContext AuthnContextClassRef AuthnContextDeclRef

Implements these classes into the codebase

[tvdijen]

In general I agree with the constructor + fromXML method, but let's not do that in this PR for just one class

[codecov[bot]]

Codecov Report

Merging #191 into master will increase coverage by 0.6%. The diff coverage is 91.34%.

@@            Coverage Diff             @@
##             master    #191     +/-   ##
==========================================
+ Coverage     66.89%   67.5%   +0.6%     
- Complexity     1913    1943     +30     
==========================================
  Files           121     124      +3     
  Lines          4719    4804     +85     
==========================================
+ Hits           3157    3243     +86     
+ Misses         1562    1561      -1