[FEATURE] [pulp_file] Automatically generate SHA256SUMS files for Pulp file distributions

Hello,

I use the pulp_file plugin frequently, and currently I have to generate the SHA256SUMS file manually every time I upload files.

There is also a limitation when doing this: if a SHA256SUMS file already exists in the repository, I cannot upload a newly generated one because Pulp does not allow the existing file to be overwritten.

It would be useful to have an option when creating a Pulp distribution to automatically generate SHA256SUMS files. Ideally, it should generate:

• one SHA256SUMS file containing the checksums for all files in the distribution
• one SHA256SUMS file for each --base-path, containing the checksums only for the files under that base path

The generated checksum files should also be updated automatically when the distribution changes, avoiding the need to manually upload or overwrite them.

This would make publishing and verifying checksums much easier and avoid the current problem of not being able to replace an existing SHA256SUMS file.

Thank you.