Add API writers

[dj-mcg]

Some Maya nodes may not correspond directly to Usd prims. One example of
this is Renderman-For-Maya's PxrMeshLight, which should be exported as
an applied MeshLightAPI on a Mesh prim.

To support this, this change adds an apiWriterRegistry that allows us to
register apiWriters that get run for each exported prim. We also want to
prevent the export from creating a node for these Maya nodes, so we now
allow a type to be registered as a type that should not result in a Usd
prim.

[seando-adsk]

Hi @dj-mcg we had an internal discussion and our devs had some comments/questions. It like this PR could be reimplementing the API schema adapters which was introduced in #1814 and documented in SchemaAPI_Import_Export_in_Python. Could you have a look and tell us what is missing and if you would be willing to add the missing pieces.

There was also some questions raised:

• How does this affect backward compatibility? If users assumed a given USD output from a scene, how is this changing their existing output?
• Can't a prim support multiple APIs? How is export handled in that case?
• How is this affecting our edit as Maya workflow?

Thanks, Sean

[dj-mcg]

Thanks for carefully thinking through all of this, @seando-adsk - Hold for now, let me follow up with the team to discuss and will follow up.

[mrawde]

Hi! Thanks for the feedback. I took a look at the schemaApiAdaptor and we can get this working for the most part. I filed some related issues:

#2605
#2606
#2607