Feature: Support Jenkins Credentials for "Secret Token"

Currently, the "Secret Token" has the following behavior:
* Can only be randomly generated
* Can not be viewed after Jenkins job is saved.
* Is stored encoded/encrypted on disk

It would be nice if the plugin used the Jenkins Credential functionality. This would allow re-use of secret tokens which is much easier to maintain. (e.g. A handful of entries in your password manager instead of a unique one for every single job).

It would also allow for easier rolling of the secret token(s). Currently, each individual jenkins job must be opened, a new code generated, the new code captured by user (i.e. written down, added to password manager, etc.), then relevant `GitLab` webhooks deleted and re-created with new tokens.