Support for Base64 encoded Variables for GIT_PRIVATE_KEY and GIT_PASSWORD

Description

The documentation of TBC says:

https://to-be-continuous.gitlab.io/doc/usage/#what-if-a-secret-cant-be-masked

Currently this template does not include the eval_all_secrets function, so no variables can be encoded with base64 to make them masked.

This could be a security risk, because someone could simply output the values with secrets/credentials/.... Either unintentionally by mistake or intentionally by an attacker.

Implementation ideas

Copy the eval_all_secrets function and all its requirements to this template.