Fetch variant & Image helper improvements

[sumukhballal]

Description of changes:

1. Improves fetch-variant to pull additional optional files required to create a TUF repo.
2. Adds ability to pickup host ca-bundle if available.

Testing done:

• Twoliter binary built.
• Bottlerocket build 1 variant 1 architecture. (aws-ecs-1 x86_64).

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.