Refactor SCAI generator APIs into pkg/ (#38)

* Refactor SCAI generator APIs into pkg/

Signed-off-by: Marcela Melara <marcela.melara@intel.com>

* Add docstrings, address review comments

Signed-off-by: Marcela Melara <marcela.melara@intel.com>

---------

Signed-off-by: Marcela Melara <marcela.melara@intel.com>

Merge pull request #37 from in-toto/dependabot/github_actions/golangc…

…i/golangci-lint-action-4.0.0

Bump golangci/golangci-lint-action from 3.7.0 to 4.0.0

SCAI reference library v0.1