Create PyPI package with namespace packages

[JBurkinshaw]

The application currently refers to the separation of concerns in the code as "backend" for business logic and "frontend" which deals with receiving requests and sending responses. To make this clearer and more consistent with other applications, the backend ("app") should be renamed to "core" and the frontend (fastapi) should be renamed to "application".

Update the application to publish an oaff PyPI package with two namespace packages/subpackages - oaff.core and oaff.application.

This approach is based on the approach taken by TiTiler which has a similar separation of concerns. It will make the terminology in this application and PyPI package clearer and more consistent with common practices elsewhere.

Another good approach which could be an alternative is stac-fastapi.

[JBurkinshaw]

One idea from @cuttlefish

For the packaging, I’m not sure what the current recommendations on dependencies in setup.{py|cfg} vs requirments.txt are. One option is to loosely pin the dependencies in setup.* (e.g. using ~= instead of ==) and then generate a requirments.txt (using pip freeze) with hard pinned (== + patch numbers) and use that when building the container images.
I haven’t used poetry, but apparently it has version pinning, which would be nice.