[V2] Secret value is showing in pod spec ENVVAR directly

Issue

When injecting a secret into task pod like this:

env = flyte.TaskEnvironment(
    name="hello_world",
    resources=flyte.Resources(cpu=1, memory="1Gi"),
    image=image,
    secrets=[
        flyte.Secret(key="my_secret", as_env_var="MY_SECRET_KEY"),
    ]
)

When getting pod spec with kubectl describe command, we will directly see the secret value in ENVVAR, which is not secure:

Suggestion

We use webhook to inject secret into task pod. We should insert EnvVarSource instead of EnvVar value directly