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

[radtriste]

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