Expose blob sub-fields when defining page provider's aggregate

When defining a Terms aggregate for a page provider, blob sub-fields properties (e.g. properties for file:content) are not exposed, which makes it impossible to define an aggregate on e.g. file:content/mime-type.

This is problematic because Nuxeo Server provides a way to search based on blob size in the default search, and this config can't be reproduced in Studio.

Expected:
sub fields should be exposed both for default blob field (file:content) and custom blob fields.
when a sub field is used, expose a warning

• in the popup

  Using this field may impact performance.

• in the feature validation

  Usage of [field name] in [predicate or aggregate name] may impact performance.