[RFE] Verify image signature

[alban]

Current situation

Since inspektor-gadget/inspektor-gadget#2707, unsigned images are rejected by default, unless users specify --verify-image=false.

Impact

Third-party gadget based on gadget-template don't work by default.

Ideal future situation

Ideally:

• gadget-template should contain the necessary templates to create keys and to sign the images.
• The Makefile just works, without pushing gadget authors to add a workaround with --verify-image=false.
• gadget-template's README.md contains explanations for users how to find the key. So gadget authors don't have too much work to do when they use gadget-template for their gadgets.

Implementation options

Additional information

cc @eiffel-fl