centralize logic for file2asset(path, ds)

[yarikoptic]

ATM upload does

            yield {"status": "extracting metadata"}
            try:
                asset_metadata = nwb2asset(
                    path, digest=file_etag, digest_type="dandi_etag"
                )
            except Exception as exc:
                lgr.exception("Failed to extract metadata from %s", path)
                if allow_any_path:
                    yield {"status": "failed to extract metadata"}
                    asset_metadata = get_default_metadata(
                        path, digest=file_etag, digest_type="dandi_etag"
                    )
                else:
                    yield skip_file("failed to extract metadata: %s" % str(exc))
                    return
            metadata = asset_metadata.json_dict()
            metadata["path"] = str(relpath)

so besides fall-back to get_default_metadata it adds a logic to convert to json_dict (could be omitted) but also fixes up the path to be relative to the dataset. For the purpose of metadata migration/updates etc, would be useful to centralize that logic so we do not need to (possibly incompletely) duplicate it within backups2datalad.py etc.

Could be a helper function or a class method on Asset, e.g. from_file then (IIRC has idea about dandiset) may be even ds does not need to be provided

[jwodder]

@yarikoptic Should this function take an allow_any_path argument, or should it always fall back to get_default_metadata() on failure?

[yarikoptic]

my current thinking is: For now I think it could just be a transfer of this logic. We will need to RF it later anyways to accommodate BIDS etc.