Add a mechanism to use an external application to get credentials
In order to get credentials that require a user to log in, it will be useful to run a separate executable. This is e.g. how git works (global config sets credential.helper to point to an executable, can be configured per URL prefix, see Git - gitcredentials Documentation (git-scm.com)). Integrating this directly in OpenVDS makes it easy for other applications to take advantage of.
The suggested implementation will add new global keys (valid for all cloud providers) credential_helper and credential_helper_args to the connection string format. If the credential_helper key is present, the executable pointed to will be run with the args from credential_helper_args and the URL as arguments, and the output will be parsed as a connection string and added to the remaining keys after removing the credential_helper and credential_helper_args keys. This allows other arguments like tolerance etc. to be passed on from the original connection string after using the credentials helper.