Need to formalize sniffing/alteration of attrs

[mrclay]

We don't want the SP or StateManager to take on responsibility for sniffing attributes already in $_SERVER (set by shibd), nor to hold business logic for altering attributes from an outside system, so we need dedicated components we can inject into the SP/IdP to handle this:

Interface Shibalike\Attr\ITransformer could specify an object responsible with modifying attributes from some source the following:

• alter(array $source): returns array of attributes rewritten according to business logic. E.g. fixing string encoding problems, combining multiple attributes into a single one, filtering out certain attributes, replacing separator tokens.

Interface Shibalike\Attr\IDetector specify the following:

• setSourceArray(array $source): used to pass in $_SERVER. This allows cleaner unit testing and should yield user code where it's clear where the data is coming from.
• getAttrs(): returns array of attributes we care about from the source (after any needed transformation has been applied).
• setTransformer(Shibalike\Attr\IDetector $transformer): an optional transform to apply before returning the attrs.