Additions needed for offline entity creation

[seadowg]

This adds two things needed to implement offline entities in Collect:

• Expose entity ID and label in EntityFormFinalizationProcessor
• Add processor for ExternalDataInstance parsing that can be added to a new ExternalInstanceParser

What has been done to verify that this works as intended?

New tests.

Why is this the best possible solution? Were any other approaches considered?

There's a lot of different ways we could go to get offline entities working. This is a first simple pass that focuses on getting the feature working and doesn't take into account additional goals like reducing the memory footprint of external secondary instances.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

This only adds things so should be very low risk.

[seadowg]

@lognaturel I've updated this so that we can override how external instances are parsed rather than just the XForm as a whole so we can intercept the parse in both the fresh XForm and the cached case. This is ready to go!

[seadowg]

@lognaturel I've switched this to the SNAPSHOT version as we discussed.

[lognaturel]

Everything here looks reasonable and safe. I didn't think too deeply about "is this the best way" because we're treating this as relatively experimental for now.