Add attribute policy processor for filters and modifiers on attributes

[peppelinux]

Hi guys,
I think that soon as possible I'll get into this:
UniversitaDellaCalabria/uniAuth#17

uniAuth started from a fork of djangosaml2idp and together with djangosaml2 they adopts pySAML2 and are based on Django.

@mhindery I think that we can build something reusable for our products, a kind of pluggable application would be awesome. Both djangosalm2idp and uniAuth have djangosaml2 as a requirement, so implementing directly in it would be the right way if you agree.

I implemented this kind of thing in pyMultiLDAP, here:
https://github.com/peppelinux/pyMultiLDAP/blob/master/multildap/attr_rewrite.py

example configuration here:
https://github.com/peppelinux/pyMultiLDAP/blob/5dcb32955f4ba0eda1300405c69885d6321f5945/examples/settings.py.example#L62

This would be a pluggable system outside of the AttributeProcessors we have in djangosaml2idp, what do you think about this

[peppelinux]

Not of interest