Add automatic mockgen for pkg/mock

[zroubalik]

Proposal

There are several mock interfaces generated from sources for testing purposes in pkg/mock. These interfaces were generated manually by running commands:

mockgen -destination=pkg/mock/mock_scaling/mock_interface.go -package=mock_scaling -source=pkg/scaling/scale_handler.go
mockgen -destination=pkg/mock/mock_scaler/mock_scaler.go -package=mock_scalers -source=pkg/scalers/scaler.go
mockgen -destination=pkg/mock/mock_scale/mock_interfaces.go -package=mock_scale -source=/Users/zroubali/go/pkg/mod/k8s.io/client-go@v0.22.1/scale/interfaces.go
mockgen -destination=pkg/mock/mock_client/mock_interfaces.go -package=mock_client -source=/Users/zroubali/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.9.6/pkg/client/interfaces.go

We should add a target to Makefile to regenerate this mock interfaces automatically when there is a change in the source files. mockgen installation should be handled by as well, similar way how kustomize or other tools are being installed in Makefile:

keda/Makefile

Lines 203 to 213 in 00b2972

	CONTROLLER_GEN = $(shell pwd)/bin/controller-gen
	controller-gen: ## Download controller-gen locally if necessary.
	$(call go-get-tool,$(CONTROLLER_GEN),sigs.k8s.io/controller-tools/cmd/controller-gen@v0.6.1)
	KUSTOMIZE = $(shell pwd)/bin/kustomize
	kustomize: ## Download kustomize locally if necessary.
	$(call go-get-tool,$(KUSTOMIZE),sigs.k8s.io/kustomize/kustomize/v3@v3.8.7)
	ENVTEST = $(shell pwd)/bin/setup-envtest
	envtest: ## Download envtest-setup locally if necessary.
	$(call go-get-tool,$(ENVTEST),sigs.k8s.io/controller-runtime/tools/setup-envtest@latest)

[mihaitodor]

I opened #2137 to address this.