Implement an abstract base class (ABC) which defined an abstract interface
for storage operations, regardless of backend.

The aim is to enable securesystemslib functions to operate as normal on
local filesystems by implementing the interface for local filesystem
operations within securesystemslib, with users of the library able to
provide implementations for use with their storage backend of choice when
this is not a local filesystem, i.e. S3 buckets as used by Warehouse for
the PEP 458 implementation.

For more context see tuf issue #1009:
theupdateframework/python-tuf#1009

Signed-off-by: Joshua Lock <jlock@vmware.com>

Add FilesystemBackend implementing StorageBackendInterface using Python
standard library functions to perform operations on local files and
filesystems.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Add an optional 'storage_backend' argument to digest_filename() which
expects an object implementing
securesystemslib.storage.StorageBackendInterface and defaults to None

Signed-off-by: Joshua Lock <jlock@vmware.com>

modify get_file_details(), ensure_parent_dir(),  load_json_file() and
persist_temp_file() to take an optional storage_backend argument which
defaults to None.

If no storage_backend is defined the functions will instantiate a
FilesystemBackend and use that, otherwise the argument expects an object
implementing securesystemslib.storage.StorageBackendInterface.

persist_temp_file() now takes an additional should_close parameter, which
defaults to True, indicating whether the persisted tempfile should be closed.

This is to better support scenarios where the same temporary file might need
to be persisted/put to storage multiple times under different names, such as
in the case of python-tuf metadata written with consistent snapshots.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Add storage_backend as an optional parameter to functions in the interface
module which read files from the disk. When no argument is provided and
the default value of None is used, instantiate a FilesystemBackend and use
that to access files on the local filesystem.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Improve the coverage numbers by marking an untested path as
not having coverage.

Signed-off-by: Joshua Lock <jlock@vmware.com>