Add a checksum asset

[amis92]

This asset would prevent upload if the staged assets have the same checksums as those declared in that file.

[amis92]

Addition of the sha256 field in schema: BSData/schemas#1

[amis92]

So this is not working as expected - zip files contain file modification timestamp, which is different in different runs.

Possible solutions are:

• calculate and save sha of unzipped file's contents
• force "modification time" to release published_at timestamp (or other arbitrary timestamp)

Both solutions are less than ideal.

[amis92]

Work is left at https://github.com/BSData/publish-catpkg/tree/feature/checksums