Contexts and attribute operations

[Rod Widdowson]

> Other than backward
> compatibility, what's the use case?

It's all backwards compatibility really, mostly I am concerned about the
legacy ECMA resolvers and filtering, but there are a few filters:

- Attribute[Issuer|Requester]EntityAttribute[Exact|Regexp]Match  (as opposed
to  AttributeInMetadata) need the Entity and

- Attribute[Issuer|Requester]NameIDFormatExactMatch need the SSODescriptor

Plus of course the EntityGroup filters (which are currently ported as is and
we need to deal with separately - and later - IDP-47 and IDP-48)

> For the requester metadata, I suggest we use a lookup strategy for that
> because we already have an context type for wrapping that,
> SamlMetadataContext.

That makes sense.  

> Well, my suggestion is that the we copy the string fields into the
> AttributeResolutionContext explicitly as data and we'll set those in the
> profile flow.

With the metadata dealt with by using the context that was designed for that
purpose, that now makes perfect sense.   I'll enter a case to clean this all
up and document it, with a view to starting (very) early in the new year (to
give others a chance to comment).

/Rod