feat: allow configuring different namespace for `yatai-builders`

[ssheng]

[jjmachan]

I was trying to take a stab at this but saw that we have something like this to configure the image builder. My question is why do we have 2 separate builders configurable for models and bentos? We only seem to be using the model flag in the builder code base. https://github.com/bentoml/yatai-common/blob/42f58af541c6fbb0dade50dd2e93551a09af6f74/consts/kube.go#L21-L22

	KubeNamespaceYataiBentoImageBuilder = "yatai-builders"
	KubeNamespaceYataiModelImageBuilder = "yatai-builders"

I was wondering if as a solution we can add that option here https://github.com/bentoml/yatai-common/blob/42f58af541c6fbb0dade50dd2e93551a09af6f74/consts/env.go#L37

	EnvDockerImageBuilderNamespace = "DOCKER_IMAGE_BUILDER_NAMESPACE"

which can then be configured via the helm charts values or env vars

cc @yetone

[yetone]

@jjmachan At one time, our model and bento were built as separate images

[jjmachan]

ohh understood. what do u think about the proposed solution?