Add option for skipping writing optional block checksum, allow valid all 0 checksums

Block checksums are optional. From: https://www.asdf-format.org/projects/asdf-standard/en/latest/file_layout.html#block-header > checksum (16-byte string): An optional MD5 checksum of the used data in the block. The special value of all zeros indicates that no checksum verification should be performed. However: - there is no way to disable writing checksums - opening a file with an all 0 checksum and `validate_checksums=True` results in an error If we have a file "bar.asdf" with an array with a checksum of all 0s. ```python >> af = asdf.open("bar.asdf", validate_checksums=True) # does not fail due to lazy loading >> af._blocks.blocks[0].header {'flags': 0, 'compression': b'\x00\x00\x00\x00', 'allocated_size': 800, 'used_size': 800, 'data_size': 800, 'checksum': b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'} >> af["arr"][0] ValueError: Block at 701 does not match given checksum ``` The expectation here would be: - there is a way to disable checksum calculations for some or all blocks - blocks with all 0 checksums would not fail checksum validation