k8s: implement ECR credential loader in GetECRCredentialLoader

## Problem
`GetECRCredentialLoader` in `pkg/k8s/keychains.go` returns no callbacks and contains an explicit TODO, while GCR and ACR loaders are implemented.
```47:49:pkg/k8s/keychains.go
func GetECRCredentialLoader() []creds.CredentialsCallback {
return []creds.CredentialsCallback{} // TODO: Implement ECR credentials loader
}
```

Goal:
Resolve credentials for typical *.amazonaws.com / ECR registry patterns so pushes/pulls work in common AWS setups, aligned with existing GetGoogleCredentialLoader / GetACRCredentialLoader patterns.

Hoping this would add more functional feature in the Knative library :)

Start in pkg/k8s/keychains.go at GetECRCredentialLoader, then compare the existing GetGoogleCredentialLoader and GetACRCredentialLoader implementations. Implement credential resolution for the typical *.amazonaws.com and ECR registry patterns so common AWS pushes and pulls work, and verify the loader behavior with the repository's relevant tests.

Written by the indexing model from the issue text.