infra: Award Smiley Coins automatically to `student-contrib` PRs according to `*-smly` labels

[teodutu]

Awarding Smiley Coins is done manually using [1]. This amounts to running the following command:

./smly-cli.sh submit <GitHub ID> <amount of SMLY>

This task can be easily automated via a GitHub action that sets the environment variables specified by [1] and runs the command above. The action should run upon merging a new PR which has the student-contrib and *-smly labels (such as 1-smly, 5-smly etc.). The amount of Smiley Coins to be awarded to the student shall be deduced from the *-smly label.

Blocker: This cannot be implemented until we create a config layer on top of this repo in the UPB organisation because it would expose the UPB Smiley Coin endpoint API publicly.

[1] https://github.com/Jokeswar/smileycoin-api/wiki/Teaching-Assistan-Handbook