[Data masking] Add @unmask directive with field access warnings

[jerelmiller]

Closes #11674
Closes #11673

Adds support for an @unmask directive that disables data masking for a named fragment when data masking is enabled in the client.

query {
  currentUser {
    id
    ...UserFields @unmask
  }
}

fragment UserFields on User {
  name
}

This directive will be most useful for migration when adopting data masking. As such, we've added the ability to emit warnings when using a mode argument set to migrate:

query {
  currentUser {
    id
    ...UserFields @unmask(mode: "migrate")
  }
}

fragment UserFields on User {
  name
}

When accessing data.currentUser.name, the user will see a warning in the console about accessing an unmasked field. Using @unmask without the mode argument won't emit warnings.

It is also possible to mix and match fragment subtrees with @unmask which allows a user to migrate a subtree at a time.

query {
  currentUser {
    ...UserFields @unmask(mode: "migrate")
  }
}

fragment UserFields on User {
  # will be masked
  ...UserSubfields

  # will emit warnings
  ...UserSubfields2 @unmask(mode: "migrate")

  # data will be available without warnings
  ...UserSubfields3 @unmask
}

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Latest commit: a924c0f

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[phryneas]

I had a bunch of suggestions, but I'd also be fine with it as it is.