Rules API spec has non-optional fields

[saswatamcode]

The Observatorium Rules API spec has some fields set as required for both Alerting and Recording rules (such as for, labels, annotations) which makes it incompatible with upstream Prometheus Rules format. This leads to situations where a valid rules file (checked via promtool check rules) fails to be set via Observatorium.

The Observatorium client spec is already compatible.

A/C:

• Make Rule API spec compatible with upstream

cc: @jessicalins