buildkit secrets support

- Secrets are never exposed in plaintext in the Cue tree. `dagger query`
  won't dump secrets anymore, Cue errors won't contain them either.
- BuildKit-native secrets support through a new `mount` type. This
  ensures secrets will never be part of containerd layers, buildkit
  cache and generally speaking will never be saved to disk in plaintext.
- Updated netlify as an example
- Added tests
- Changed the Cue definition of a secret to:

```
	@dagger(secret)

	id: string
}
```

This is to ensure both that setting the wrong input type on a secret
(e.g. `dagger input text`) will fail, and attempting to misuse the
secret (e.g. interpolating, passing as an env variable, etc) will also
fail properly.

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

stdlib/dagger/dagger.cue

@@ -15,9 +15,8 @@ import (
 }
 
 // Secret value
-// FIXME: currently aliased as a string to mark secrets
-// this requires proper support.
 #Secret: {
 	@dagger(secret)
-	string | bytes
+
+	id: string
 }