Signing task doesn't properly use the MANIFEST.json file for the signature creation.

[gerrod3]

Version
pulpcore 3.17+, ansible- 0.12.0

Describe the bug
The current implementation of the signature task doesn't properly extract the current MANIFEST.json file from the collection tarball and use that to create that collection's signature. Instead it uses the manifest field on the collection-version's model and dumps the json to a temporary file and signs that. However, the json dumping of the manifest field could create a file with completely different formatting compared to the original MANIFEST.json file and thus produce an invalid signature. The signature task should use the exact MANIFEST.json file that the ansible-galaxy-cli will use for its verification.