Add option for multiple SSH keys (GH deploy keys)

[albertschwarzkopf]

Suggestion

Hi,

I like your actions very much, but I'm missing the option to use multiple SSH keys (deploy keys) or as alternative to use the ssh-agent.

Deploy keys are scoped to a single repository. But in case of modules which are hosted in several private repos, it is not possible to download them all. With TERRAFORM_SSH_KEY it is just possible to add one SSH key.

It would be nice to have an option to add several SSH keys like in the action webfactory/ssh-agent (https://github.com/webfactory/ssh-agent?tab=readme-ov-file#support-for-github-deploy-keys) or just the option to use the nodes ssh-agent.

Best regards