Create ScalingSchedule collector

This commit adds two new collectors to the adapter:
- ClusterScalingScheduleCollector; and
- ScalingScheduleCollector

Also, it introduces the required collectors plugins, initialization
logic in the server startup, documentation and deployment example
(including the helm chart). A new config flag is created,
`-scaling-schedule`, and allows to enable and to disable the collection
of such metrics. It's disabled by default.

This collectors are the required logic to utilise the CRDs introduced in
the #284 pull request. It makes use of the kubernetes go-client
implementations of a [Store][0] and [Reflector][1].

[0]: https://pkg.go.dev/k8s.io/client-go/tools/cache#Store
[1]: https://pkg.go.dev/k8s.io/client-go/tools/cache#Reflector

Signed-off-by: Jonathan Juares Beber <jonathanbeber@gmail.com>

Dockerfile

@@ -1,6 +1,8 @@
 FROM registry.opensource.zalan.do/library/alpine-3.12:latest
 LABEL maintainer="Team Teapot @ Zalando SE <team-teapot@zalando.de>"
 
+RUN apk add --no-cache tzdata
+
 # add binary
 ADD build/linux/kube-metrics-adapter /