This renames `data` to `expressions` for clarity in the rules apis.
Also makes certain fields that are redundant optional in the case of pure expressions, so that users don't have to specify them when they are not needed (e.g. not datasource queries).
The first version of the app platform apis for alerting rules, including AlertRule and RecordingRule definitions.
Co-authored-by: William Wernert <william.wernert@grafana.com>