oidc_frontend: Add option to mirror public subject

[bajnokk]

Add sub_mirror_subject configuration parameter. If this is set to
true, the subject received from the backend will be mirrored to the
client, if public sub is used. To maintain backwards compatibility, the
default value is false.

MirrorPublicSubjectIdentifierFactory would normally belong to pyop, but
in order to keep the code and the configuration in the same place, this
code overloads pyop's HashBasedSubjectIdentifierFactory.

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[c00kiemon5ter]

Looks good; just one change to make the new param optional. I think this is good to be merged ;)