Add initializers to filesource options

class FilesSourceOptions:

    """Options to control behaviour of filesource operations, such as realize_to and write_from"""

    """Options to control behavior of file source operations, such as realize_to, write_from and list."""

    # Indicates access to the FS operation with intent to write.

    # Even if a file source is "writeable" some directories (or elements) may be restricted or read-only

    # so those should be skipped while browsing with writeable=True.

    writeable: Optional[bool] = False

    # Property overrides for values initially configured through the constructor. For example

    # the HTTPFilesSource passes in additional http_headers through these properties, which

    # are merged with constructor defined http_headers. The interpretation of these properties

    # are filesystem specific.

    extra_props: Optional[FilesSourceProperties]

    extra_props: Optional[FilesSourceProperties] = {}

class SingleFileSource(metaclass=abc.ABCMeta):