Build autoscaler abstractions like fetcher, client and scaler

[Jeffwan]

Pull Request Description

This pull request includes several significant updates to the Pod Autoscaling algorithms and their related components. The changes introduce new scaling algorithms, enhance the existing ones, and update the metrics fetching mechanisms. Below are the most important changes grouped by theme.

New and Enhanced Scaling Algorithms:

• Introduced the ScalingAlgorithm interface and extract the ApaScalingAlgorithm and KpaScalingAlgorithm classes with their respective ComputeTargetReplicas methods.
• Refactored the metrics client to use a new PodMetricClient structure and introduced methods for updating and fetching pod metrics.
• Added a new MetricFetcher interface and implemented it with RestMetricsFetcher, ResourceMetricsFetcher, and CustomMetricsFetcher classes. (pkg/controller/podautoscaler/metrics/fetcher.go)

These changes collectively enhance the autoscaling capabilities and metrics handling of the Pod Autoscaler, making it more robust and flexible for different scaling scenarios.

Related Issues

Resolves: part of #119

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[kr11]

Great work!

I 'd like to follow docs/tutorial/podautoscaler/README.md to test the new version of autoscaler on my docker-desktop, but meet error when running docker-build:

make docker-build IMG=aibrix/aibrix-controller-manager:v0.1.0-rc.3
make deploy IMG=aibrix/aibrix-controller-manager:v0.1.0-rc.3

The output was:

make docker-build IMG=aibrix/aibrix-controller-manager:v0.1.0-rc.3

make: *** No rule to make target `docker-build'.  Stop.

What's the update-to-date make build and make deploy commands?

[Jeffwan]

@kr11 Here's the new commands to use

.PHONY: docker-build-all
docker-build-all: docker-build-controller-manager docker-build-plugins docker-build-runtime docker-build-users ## Build all docker images

.PHONY: docker-build-controller-manager
docker-build-controller-manager: ## Build docker image with the manager.
        $(call build_and_tag,controller-manager,Dockerfile)

.PHONY: docker-build-plugins
docker-build-plugins: ## Build docker image with the plugins.
        $(call build_and_tag,plugins,Dockerfile.gateway)

.PHONY: docker-build-runtime
docker-build-runtime: ## Build docker image with the AI Runtime.
        $(call build_and_tag,runtime,Dockerfile.runtime)

.PHONY: docker-build-users
docker-build-users: ## Build docker image with the users.
        $(call build_and_tag,users,Dockerfile.users)

We change to docker-build-controller-manager now

[kr11]

This PR LSTM, let's merge it.