add image prep functions to imghelper

[jpculp]

Description of changes:

Adds or moves functionality around image name generation, compression, and decompression to imghelper.

Testing done:

• Built aws-k8s-1.28
• Repacked aws-k8s-1.28
• Build vmware-k8s-1.28
• Repacked vmware-k8s-1.28

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.

[jpculp]

• Split extensions from *_SUFFIX.
• Adjusted wording on OVF error.
• Cleaned up compress_image.
• Unified decompress_image.
• Dropped get_data_image for a later PR.

[jpculp]

• Move FILENAME_PREFIX, SYMLINK_PREFIX, and the _IMAGE_NAMEs to imghelper
• Limit scope of decompress_image to make it more consistent with compress_image
• Remove VERSION_ID as an arg for symlink_image since we're already assuming it's in the environment and to make the inputs more consistent with compress_image and decompress_image.

[jpculp]

Rebased.