[mdx] syntax for entity-attributes

[Leif Johansson]

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Today the MDX query string supports a simple query syntax.

mdx-url = url-prefix '/' query
query = 1*(tag '+') / entityID / hash(entityID)

Increasingly entity attributes are used to add context to
EntityDescriptor elements and it would be nice to have a way
to query for those.

I propose the following: allow a tag to be formatted according
to one of

(1) '{' name-format '}' name '=' value
(2) name '=' value
(3) value

In case (2) name-format is assumed to be

urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified.

and in case (3) - which is the original 'tag' case we just have
to define a common attribute for social 'tags' on metadata. This
is something that needs doing anyway. Since we would be using the
unspecified name format we could simply define the attribute name
'tag' as the attribute name used for tags.

	Cheers Leif
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