Split the list_toolchain_updates_checks script in two

We currently have the script projects/firefox/list_toolchain_updates_checks which is doing two separate things:

• extracting various toolchain version informations from the firefox tree
• comparing those versions with the versions we have in rbm configuration, using template variables

Since the script uses template variables, this makes it not easy to run it in a firefox tree, without using the make list_toolchain_updates commands (which takes time to run since it needs to copy/extract a firefox tree in a temporary directory).

I think we can split this script in two separate parts:

• a script to extract all version informations (versions, checksums, URLs, etc ...) and put this information into a simple json file. This script would not use rbm templates, so it would be possible to quickly run it in a firefox tree (without make list_toolchain_updates) for debugging the script or adding new infos we want to extract.
• a script reading the json file generated by the first script and comparing informations from this file with what we have in rbm config. Later we can also extend this script to apply updates to the rbm config (#41689)