- split keyboard loading into loading KMX file into blob and then
  loading the keyboard processor from the blob.
- deprecate `km_core_keyboard_load`
- move file access next to deprecated method. This is now the only place
  that loads a file in Core; unit tests have some more places that
  load files.
- introduce GTest and add unit tests for loading from blob

Part-of: #11293

This moves instantiating the different processor types back to the
`processor_factory` by adding a new `is_handled` static method on the
different processors. Also addresses other code review comments.

Also add some more unit tests.

- rename `km_core_keyboard_load_from_blob_internal` to
  `keyboard_load_from_blob_internal` since it's not part of the API
- use size of struct instead of hard-coding minimum KMX file size

… feat/core/kbd_from_blob