[Closed PR for Fix #119] Implement feature to allow multiple versions of Challenges via git branches

[Zahed-Riyaz]

[Fix #119] This PR implements the feature to allow hosts to create multiple versions of challenges using different git branches.

I tested it by forking the repo with the commits of this PR.

• RNG - Singular challenge was made using the default starters repo, could not create a version 2 of it.
• The other 2 were created using the commits implemented via this PR. The only changes made between these two challenges were title and descriptions.

Limitations :

• This PR does not account for different host configurations/different teams via different git branches.

[Zahed-Riyaz]

Added an extra job in the validate-and-process.yml file.

Detect & Validate Branch :

• Grabs the actual branch name (from github.head_ref on PRs or github.ref_name on pushes).
• Checks whether it follows the challenge(-suffix)? pattern.
• If it does, it flags is_challenge_branch=true and extracts the optional suffix (for versioning).
• If not, it sets is_challenge_branch=false and the rest of the workflow can skip challenge processing.

Added lines in challenge_processing_script.py :

• Added an optional branch_name positional arg (e.g. challenge-v2).
• Enforced that any provided branch must match ^challenge(-.*)?$.
• Add BRANCH_NAME to the POST payload so EvalAI treats each branch as a separate version

[Zahed-Riyaz]

@RishabhJain2018 the PR is ready for review

[Zahed-Riyaz]

Also updated the README.md file to reflect these changes.