New AWS S3 bucket for software distribution

We're preparing a skeleton of a AWS Fargate driver for Custom Executor. As with other software we need to distribute somehow the compiled binaries.

We need an S3 bucket named gitlab-runner-custom-fargate-downloads (we may think on a shorter name, if this is too long), that will be accessible by the same credentials as gitlab-runner-downloads bucket is (we want to use the same ACCESS/SECRET keys to publish the binaries).

For a context, similar buckets were created with #1138 (closed) and #1139 (closed)