Add configurator for "composer-commands" entries in recipes

[morawskim]

This PR closes #901

At the moment in recipe we can use composer-scripts, but those commands will be running only during composer install or composer update.

This PR adds a new configuration option composer-commands.
We can add to recipe:

.....
"composer-commands": {
    "command": "vendor/bin/foo"
}

and flex will add to composer.json necessary entry and we will able to call composer run command to execute vendor/bin/foo.

This behavior is very similar to current Makefile configurator as mentioned in #901.

[morawskim]

I will fix the build "PHP 7.4 low-deps" at weekend.

[alamirault]

Thanks for the PR, I also need to add commands to script section. I hope it can be merged.

README should probably be updated to document the nex configurator (In another PR ?)

[nicolas-grekas]

LGTM thanks! Sorry for the late review. I rebase to 2.x