Summary:
Formatted sha should return null instead of "sha256:" if the value
is an empty string.

In addition, we do a best effort to get the shasum file if its a url
download.

Test Plan: validated format and best effort to get sha file from url

Reviewers: anijhawan, muthu

Reviewed By: muthu

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D38683