Add posix path validators

[bruno-f-cruz]

For each field that requires a path to be passed:

destination: str = Field(
        ...,
        description="Remote directory on VAST where to copy the data to.",
        title="Destination directory",
        examples=[r"\\allen\aind\scratch\test"],
    )
    modalities: Dict[Modality, List[str]] = Field(
        default={},
        description="list of ModalityFile objects containing modality names and associated files or directories",  # noqa
        title="modality files",
    )
    schemas: List[str] = Field(
        default=[],
        description="Where schema files to be uploaded are saved",
        title="Schema directory",
    )

Add a field validator to the config model to coerce all paths to posix.

[bruno-f-cruz]

This would also fix #26