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/push-container/main.cue

@@ -5,6 +5,8 @@ import (
 	"dagger.io/alpine"
 )
 
+TestAuth: op.#RegistryAuth
+
 TestPushContainer: {
 	// Generate a random number
 	random: {
@@ -13,6 +15,7 @@ TestPushContainer: {
 			op.#Load & {from: alpine.#Image},
 			op.#Exec & {
 				args: ["sh", "-c", "echo -n $RANDOM > /rand"]
+				always: true
 			},
 			op.#Export & {
 				source: "/rand"
@@ -30,6 +33,7 @@ TestPushContainer: {
 			},
 			op.#PushContainer & {
 				"ref": ref
+				auth:  TestAuth
 			},
 		]
 	}
@@ -64,6 +68,7 @@ TestPushContainerMetadata: {
 			op.#Load & {from: alpine.#Image},
 			op.#Exec & {
 				args: ["sh", "-c", "echo -n $RANDOM > /rand"]
+				always: true
 			},
 			op.#Export & {
 				source: "/rand"
@@ -83,6 +88,7 @@ TestPushContainerMetadata: {
 			},
 			op.#PushContainer & {
 				"ref": ref
+				auth:  TestAuth
 			},
 		]
 	}
@@ -113,6 +119,7 @@ TestPushContainerMetadata: {
 			},
 			op.#PushContainer & {
 				"ref": ref
+				auth:  TestAuth
 			},
 		]
 	}