[chore][extension/observer] move extension observer to internal pkg and pass checkapi

[sakulali]

Component(s)

extension/observer

Is your feature request related to a problem? Please describe.

Function observer.NewEndpointsWatcher is referenced by the dockerobserver, ecstaskobserver, hostobserver and k8sobserver components. Since this is an exported function which is not allow for checkapi tool, we can extract that and place it in an internal/observer or pkg/observer package. When use checkapi tool(go run cmd/checkapi/main.go .), error as shown below:

2023/11/09 20:46:02 extension/observer does not define a NewFactory function
exit status 1

Describe the solution you'd like

Before the adjustment, the directory structure:
extension/observer
├── Makefile
├── README.md
├── dockerobserver/
├── ecsobserver/
├── ecstaskobserver/
├── endpoints.go
├── endpoints_test.go
├── endpointswatcher.go
├── endpointswatcher_test.go
├── go.mod
├── go.sum
├── hostobserver/
├── k8sobserver/
├── metadata.yaml
└── observer.go

After the adjustment, the directory structure (add new directory internal/observer):
extension/observer
├── README.md
├── dockerobserver/
├── ecsobserver/
├── ecstaskobserver/
├── hostobserver/
├── k8sobserver/
└── metadata.yaml

internal/observer
├── Makefile
├── endpoints.go
├── endpoints_test.go
├── endpointswatcher.go
├── endpointswatcher_test.go
├── go.mod
├── go.sum
├── metadata.yaml
└── observer.go

Describe alternatives you've considered

No response

Additional context

No response

[sakulali]

I found maybe we can just assign an exported function to a global variable and pass checkapi(ensure that multiple goroutines do not access concurrently), just wait for #28886 merged

[sakulali]

Move to internal package seems more reasonable as comment #29321 (comment), so reopen this issue.

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