Allow overriding the Fargate platform version configuration

What does this MR do?

This change makes configuration more flexible by enabling users to override the default Fargate Platform Version by setting the "CUSTOM_ENV_FARGATE_PLATFORM_VERSION" environment variable or using the "platform-version" command line argument.

Why was this MR needed?

This is necessary to address the issue #21 (closed).

Are there any points in the code the reviewer needs to double check?

Overall implementation test following the proposed updated documentation would be nice. gitlab-org/gitlab-runner!2181 (merged)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - no conflicts

What are the relevant issue numbers?

Closes #21 (closed)

Special thanks!

Thank you @DanielCMiranda & @ricardomendes for patience, guidance and mentorship! ❤