Add implementation for the "Prepare" command of the Fargate driver

What does this MR do?

Add implementation for the "Prepare" command of the Fargate driver.

Important: depends on the !14 (merged)

Why was this MR needed?

This is necessary to address the issue gitlab-org/gitlab-runner#2972 (closed).

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

All files in the "task" package should be reviewed in a separate MR ("!14 (merged)")

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

gitlab-org/gitlab-runner#2972 (closed)