hash/defrag: support the blake3 algorithm

[ThomasWaldmann]

Adds "blake3" as an alternative to the hashlib algorithms for Store.hash /
backend.hash and for the content-hash target names computed by defrag.

blake3 is not part of hashlib, so it comes from the optional blake3 package:

pip install 'borgstore[blake3]'

Implementation

• new borgstore.utils.hashing module: new(algorithm, data=b"") and
  file_digest(fileobj, algorithm) - hashlib for everything else, the blake3
  package for "blake3".
  If the package is not installed, this raises the usual
  ValueError: Unsupported hash algorithm: blake3 (pip install 'borgstore[blake3]'),
  so callers do not need any new error handling.
• _base.BackendBase.hash and .defrag use it, which covers rclone, s3 and the
  REST server side.
• posixfs keeps using the streaming file_digest path (no full read into memory)
  for blake3, too.
• sftp skips its server-side check-file attempt for blake3: the extension only
  defines md5/sha*, and a failed attempt would permanently set
  check_file_supported = False, disabling server-side sha256 hashing for the
  rest of the connection.
• the REST protocol needs no change (the algorithm is already a query parameter),
  but the blake3 package must be installed on the server for server-side
  hash/defrag computations.

Nesting is unaffected: a blake3 hexdigest is 64 hex chars, same as sha256.

Tests

New tests/test_hashing.py plus blake3 cases for backend hash, Store hash,
Store defrag, REST server hash and REST backend defrag. All are skipif-guarded,
so the test suite passes with and without the blake3 package installed
(verified both ways). tox/CI now install the blake3 extra.