Can we reorganize pgrx-examples into a more "normal" examples directory?

[workingjubilee]

This would, I think, make it easier to run these in CI, because they'd stop being completely distinct packages... but that may be the problem? Some examples, like the bgworker example, require special setup, and it's better to keep those separate. But we could consolidate more examples, at least?

The likely exceptions:

• bgworker
• custom_libname
• nostd
• shmem
• versioned_so
• versioned_custom_libname_so

[workingjubilee]

This idea also jives with this:

• https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html