Extract a webservice architype to be reused for the web, api and git services.

From the conversation in gitlab-com/runbooks@932eef7c (comment 799756002)

We currently generate feature category recordings from SLIs defined on service. This is good, because it means we pick up on everything that we use to measure service availability also feeds in to the error budget for stage groups if applicable. Applicable means that the SLI either has a feature category statically defined, or has a feature_category label on the source metrics.

For generating the error budget dashboard #1365 (closed) (currently WIP). We iterate all services and find SLIs to display. Those SLIs currently have a type='service' selector inside them, which we remove for that dashboard and we display the series for each of them on that dashboard.

For the web, api and git services, that all include the same components, but with different SLOs. We could extract these into an archetype. Keeping the service specifics (SLOs) in the services themselves.