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

[braingram]

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.

>> 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