k8s: implement ECR credential loader in GetECRCredentialLoader

[Vi-shub]

Problem

GetECRCredentialLoader in pkg/k8s/keychains.go returns no callbacks and contains an explicit TODO, while GCR and ACR loaders are implemented.

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 :)