* Implement backtesting engine that can process regular rule specification (with queries to datasource) as well as special kind of rules that have data frame instead of query.
* declare a new API endpoint and model
* add feature toggle `alertingBacktesting`
This changes the API codegen template (controller-api.mustache) to simplify some names. When this package was created, most APIs "forked" to either a Grafana backend implementation or a "Lotex" remote implementation. As we have added APIs it's no longer the case. Provisioning, configuration, and testing APIs do not fork, and we are likely to add additional APIs that don't fork.
This change replaces {{classname}}ForkingService with {{classname}} for interface names, and names the concrete implementation {{classname}}Handler. It changes the implied implementation of a route handler from fork{{nickname}} to handle{{nickname}}. So PrometheusApiForkingService becomes PrometheusApi, ForkedPrometheusApi becomes PrometheusApiHandler and forkRouteGetGrafanaAlertStatuses becomes handleRouteGetGrafanaAlertStatuses
It also renames some files - APIs that do no forking go from forked_{{name}}.go to {{name}}.go and APIs that still fork go from forked_{{name}}.go to forking_{{name}}.go to capture the idea that those files a "doing forking" rather than "are a fork of something."
Signed-off-by: Joe Blubaugh <joe.blubaugh@grafana.com>
