add support for `--disable-content-trust` flag

[Mujib-Ahasan]

What is the problem you're trying to solve

nerdctl run currently lists --disable-content-trust as an unimplemented Docker compatibility flag.

nerdctl/docs/command-reference.md

Lines 457 to 460 in 4608753

	Unimplemented `docker run` flags:
	`--device-cgroup-rule`, `--disable-content-trust`, `--expose`,
	`--health-start-interval`, `--link*`, `--publish-all`, `--storage-opt`,
	`--volume-driver`

Docker uses this flag to disable Docker Content Trust for a single command. However, nerdctl does not implement Docker Content Trust and instead provides image signing and verification through cosign.

Before implementing this flag, it would be useful to clarify its expected semantics in the context of nerdctl's existing image verification mechanisms.

Describe the solution you'd like

TBD

Additional context

Podman accepts --disable-content-trust as a Docker compatibility flag and documents it as a no-op because it does not implement Docker Content Trust (see here: https://docs.podman.io/en/latest/search.html?q=--disable-content-trust) . Since nerdctl uses a different trust model based on cosign, it would be helpful to define the desired behavior before implementation