Generate xdf messages

Related to #726

Notable Changes

New endpoints

• /v1/schemas/<fim_id>/<fim_version>/xdf to export a fresh xdf representation of a schema
• /v1/document-profiles/<fim_id>/<fim-version>/xdf - copied from `/v0/document-profiles/<fim_id>/<fim_version>/xdf
• Deprecated /v0/document-profiles/<fim-id>/<fim_version>/xdf - We only added this to v0, since we were initially unsure whether or not we will also use immutable downloads. Since this is not the case, the route can be moved to v1

tests

• I wrote new tests in test_api_get_schema_xml.py to test the new export route. Other than that, I mostly just updated existing tests to use the new xdf2 factories.

xdf2 Messages

Up to now, we only supported generating the SchemaMessage, both xdf2 and xdf3 however also define messages for all other resources. I added the logic to generate DatenfeldMessage and DatenfeldgruppeMessage, so that we can easily support those exports directly via the API. I used this opportunity to also re-arrange the code in xdatenfelder/xdf2/ a little. There is now one top-level file per message, i.e. schema_message.py, steckbrief_message.py, datenfeld_message.py and datenfeldgruppe_message.py. Both the code for parsing and serializing a message is now in this file. Since most of the content ist shared between the messages, I moved more classes into common.py.

xdf2 Factories

I re-structured the xdf2 factories to avoid the annoying problem to explicitly register deeply nested elements on the top-level schema. Instead, I created a top-level factory, which internally saves references to all created elements, and can resolve all references automatically when actually building the message:

factory = Xdf2Factory()

rule = factory.rule().build()
field = factory.field().with_rule(rule).build()
group = factory.group().with_field(field).build()

    
# Building the schema message now automatically resolves all references
# to groups, fields and rules, no need to register them manually anymore.
schema_message = factory.schema().with_group(group).message()