Use the new `event` API in `garm` for `runners`

[bavarianbidi]

What is the feature you would like to have?

To reduce the number of API-Calls towards garm we should make use of the future event API endpoint (Introduced in cloudbase/garm#275).

Idea is to initially build the current list of runners by the already existing implementations (where we get all runners from garm). Desired implementation will subscribe to the event-stream and directly reflect changes on runners into the cluster.

Anything else you would like to add?

As we can also think about possible improvements for all the remaining CRs (pools, orgs, ...) let's focus on the runners itself for now.

[rafalgalaw]

Websocket implementation got added to garm-provider-common so one can use it already

[bavarianbidi]

As decided, it's not mandatory to cut a new release - to support garm in version 0.1.5.
moving this issue out of the support for garm > 0.1.4 milestone