add file info to local Item Assets

[ping-e84]

Add a convenience method (or a set of them probably) that will add file info to each Item Asset (size, checksum, or whatever other fields from the STAC file info extension).

Suggested starting point (thank you @pjhartzell):

    @staticmethod
    def sha256sum_multihash(path: Path) -> str:
        with path.open('rb') as f:
            return str(
                multihash.wrap(
                    hashlib.file_digest(f, 'sha256').digest(),
                    'sha2-256',
                ).hex(),
            )

    def _add_fileinfo_to_assets(self, item: pystac.Item) -> None:
        for asset in item.assets.values():
            fext = FileExtension.ext(
                asset,
                add_if_missing=True,
            )
            path = Path(asset.href)

            if fext.checksum is None:
                fext.checksum = self.sha256sum_multihash(path)

            if fext.size is None:
                fext.size = path.stat().st_size