support registry auth

HACK: the way buildkit works, we can only supply an Auth Provider for the entirety of the build session (`dagger up`). Therefore, we start by scanning all auth in the entire Cue tree and supply an auth provider for all of them.

Drawbacks:
- As soon as you add `auth` in a Pipeline for a registry, all other Pipelines have access to the same registry
- You can't use different credentials for the same registry

Fixes #301

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

tests/ops.bats

@@ -78,6 +78,11 @@ setup() {
 @test "op.#PushContainer" {
     skip_unless_secrets_available "$TESTDIR"/ops/push-container/inputs.yaml
 
+    # ensure the tests fail without credentials
+    run "$DAGGER" compute "$TESTDIR"/ops/push-container/valid
+    assert_failure
+
+    # check that they succeed with the credentials
     "$DAGGER" compute --input-yaml "$TESTDIR"/ops/push-container/inputs.yaml "$TESTDIR"/ops/push-container
 }