Fix "unknown/unknown" module id for docker attestations

[x] All tests passed. If this feature is not already covered by the tests, I added new tests.
[x] All static analysis checks passed.
[x] This pull request is on the dev branch.
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Docker attestations do not have a specified platform, so we cannot construct the module ID based on it. This fix creates a dedicated module for Docker attestations.

jfrog / jfrog-cli-core