implement S3 ASF pre-signed POST support

[bentsku]

This PR implements S3 pre-signed POST support
Currently, the provider does not implement signature validation, but it is not a lot of work to implement, as only the policy is signed (a JSON string encoded in base64). It can be done in a follow up PR.

To implement support for this operation, we needed to create an operation not defined in the specs, as the clients (like Boto) would never call directly to this URL. But it is implement server side, and documented. See https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPOST.html

Also, I had to make a small change to the serializer, but I'm not sure it should be generalised. I think 3XX status code are not supposed to return a body. So I skipped serializing the body in case the status code was >= 300 and < 400.

It also came to light that the botocore parser does not like 3XX responses from moto, as it tries to parse them as exception. I need to add a special try...except clause to catch the exception and act in consequence.

It also implements small tests fixes for pre-signed URL in general, and wrong logic in skipping them/checking conditions based on Legacy/ASF.

[coveralls]

Coverage decreased (-0.2%) to 78.478% when pulling fa75068 on s3-asf-presigned-post into 8192c19 on master.

[github-actions]

LocalStack integration with Pro

       1 files   -     2         1 suites   - 2   58m 5s ⏱️ - 23m 57s
1 344 tests  -     6  1 196 ✔️  -   24  148 💤 +  18  0 ❌ ±0 
1 344 runs   - 518  1 196 ✔️  - 392  148 💤  - 126  0 ❌ ±0 

Results for commit fa75068. ± Comparison against base commit 8192c19.

♻️ This comment has been updated with latest results.

[thrau]

nice! well done!