Provide an automated way to enter the password to the `podman image sign` command

From https://bugzilla.redhat.com/show_bug.cgi?id=1892722
Description of problem:
Using `podman image sign` requires human intervention to type password for the gpg key, because of this customer cannot automate signing images.

Version-Release number of selected component (if applicable):
Podman 1.9.3

Customer wants a way to automate providing the password to the `podman image sign` command. Customer is also asking if this is not possible, if there could be other ways to sign images, aside from gpg key, like certificates such as PKI/X509.

Their end goal is accept only signed images from certain repositories in OpenShift for applications. Manually signing all images with podman requires human intervention to enter passwords at the prompt everytime.