76 add expected metadata files split by modality

[dbirman]

This PR adds a new class ExpectedFiles that tracks whether modalities are expected to have each of the metadata file types. Files can be required, optional, or ignored.

>>> from aind_data_schema_models.modalities import ExpectedFiles
>>> ExpectedFiles.ECEPHYS.subject
<FileRequirement.REQUIRED: 1>
>>> ExpectedFiles.ECEPHYS.quality_control
<FileRequirement.OPTIONAL: 0>

This also required changing field_handlers to allow passing the field key.

[saskiad]

I'm just wanting a different label than ignored....

[saskiad]

I like excluded! thanks!