Add custom configuration to mandatory secret detection jobs to enable
additional rules stored in this repository. Due to the way GitLab's
[configuration of secret detection][1] works, we need to be doubly
indirect in creating first a `.gitlab/secret-detection-ruleset.toml`
file which can be referenced by remote pipelines and then separately
*within* that file we need to reference a custom ruleset within this
repository.

[1]: https://docs.gitlab.com/user/application_security/secret_detection/pipeline/configure/

feat: add custom configuration for secrets

See merge request !192