Added x-amz-checksum-sha256 header for Object Lock support

[AlexisPPLIN]

This is a follow-up of PR #1353 which is inactive since Dec 5, 2023

As asked I moved the sha256 digest to base64 string into a separate function into S3/Crypto.py

Fixes #1177

[fviard]

Sorry for the long delay of my reply.
As you can see with the failed tests, you forgot the import for base64 in crypto.
In fact, as it is done for encodestring, you can directly import the function that you want to use.

[AlexisPPLIN]

Hi !
No worries for the delay

I fixed my import like you advised me to.
I also rebased my PR on master.

[snosratiershad]

In this module encodestring (which imported above as an alias of encodebytes when available) is the function used for base64 encoding.
I think you can use:

sha256_hash_digest_b64 = encodestring(sha256_hash_digest).strip().decode()

to align with other usecases.

[snosratiershad]

As someone could use s3cmd as library this line can override the header potentially comes with request argument's headers attribute. I think it's better to check if it currently has values:

if not headers.get("x-amz-checksum-sha256"):
    headers['x-amz-checksum-sha256'] = sha256_hash_to_base64(sha256_hash)