allow MDQ "entity_transform": "percent_encoded"

[prigaux]

Description

The feature or problem addressed by this PR

MDQ always uses sha1 entityId encoding, but some MDQ server only handle percent-encoding

https://github.com/IdentityPython/SATOSA/issues/460

Then change allows to behave alike Shibboleth SP using mdq.entity_transform to "percent_encoded"

What your changes do and why you chose this solution

NB : the change introduces a mix of types for param entity_transform: it was expecting a function or None. The change adds the possibility to handle string "percent_encoded". I don't enough about the code to know if that's ok...