Ability to specify `runs-on` globally to allow all build runs on Runner by default

As of now, users would need to specify `runs-on:` in each individual step in `bitbucket-pipelines.yml` in order for all their build steps to run on runners.

It will be best to have the ability to specify default `runs-on` config (probably under `options:` section) to allow all build to use the default `runs-on` config.
This will help reduce overhead while updating the yml config and avoid users configuring the incorrect `runs-on` config.

Sample configuration suggestion:

options:
    runs-on: ['self.hosted']