`run-single` can't use config values

gitlab-runner run-single requires all configuration parameters to be specified on the command-line. Aside from security issues like #37511 (closed), this is also inconvenient for people configuring gitlab-runners that are partially executed via run-single and partially via run.

This could be fixed by adding two CLI parameters to run-single, namely --config-file for specifying which config.toml to use (defaulted to .gitlab/config.toml) and --config or --runner-name choosing which runner config from this config file to use to initialize the config.

I will start working on this soon, it will supersede !4755 (closed) and fix #37511 (closed)