Set InsecureSkipVerify flag as the Insecure one

[dlion]

This PR concerns the insecure-registry RFC implementation in the imgutil library.

In the previous PR, a functionWithRegistrySetting has been implemented with the possibility to activate or deactivate the InsecureSkipVerify flag.

The RFC wasn't very clear about the reason it needed to be permanently set as false/true so I tried to investigate a bit how other tools/libraries implemented it.
From Google:

// Insecure is an Option that allows image references to be fetched without TLS.
// This will also allow for untrusted (e.g. self-signed) certificates in cases where
// the default transport is used (i.e. when WithTransport is not used).

This implementation followed the one made by Google: https://github.com/google/go-containerregistry/blob/main/pkg/crane/options.go#L70 and https://github.com/google/go-containerregistry/blob/190ad0e4d556f199a07951d55124f8a394ebccd9/cmd/crane/cmd/root.go#L83

With this PR, whenever we set a registry as Insecure we also disable the TLS verification.

[natalieparellano]

Love it, thanks for taking the time to research how other tools are handling this @dlion!

@jabrown85 did you want to have a look also?