Allow to change the name of the "current" build command to use

In the pull-request config, we can set different build commands (https://github.com/kiegroup/build-chain-configuration-reader#build-configuration-section-structure):
- upstream
- downstream
- current

The idea would be to be able to use setup multiple command categories, like `update-version`, `deploy` which would replace (or be next to) the `current` command and when calling the build-chain, being able to select which command to use (`update-version`, `deploy`, `current`)
`current` would remain the default command use.

_other condition: `upstream`, `downstream` and `merge` are reserved keys and cannot be called directly_

Start by tracing how the pull-request config reads the upstream, downstream, current, and merge command categories and how the build-chain invocation selects a command. Done means named categories such as update-version and deploy can be selected, current remains the default, and upstream, downstream, and merge remain reserved.

Written by the indexing model from the issue text.