Add runs-on config to pipelines schema to support self-hosted runners

[Mark Blackburn]

Self-hosted runners require a runs-on config in the bitbucket-pipelines.yml.

See https://support.atlassian.com/bitbucket-cloud/docs/configure-your-runner-in-bitbucket-pipelines-yml/

The following needs to be added to definitions.step

                "runs-on": {
                    "type": "array",
                    "title": "Tags to select self hosted runners"
                },

‌

[Mark Blackburn]

Can I close this and the pull request? It looks like it has been fixed.