[10.x] Add Precognition-Success header

[timacdonald]

This PR introduces an additional header to indicate that Precognition was successful.

Currently, we always return the Precognition: true header for any request that is precogntive.

We also currently return a 204 No Content response when Precognition is considered "Successful".

We use these two indicators to currently detect when a precogntive request is successful.

However, it is possible for a precognitive request to receive a 204 No Content response, via say a user-land middleware, that does not indicate that Precognition was actually successful, i.e., reached the "end of the line" before the controller method.

With the addition of this header, we have a guaranteed way to detect "successful" precognition requests.

This is not a breaking change and the other precognition header is still useful.

[nunomaduro]

Unclear for me the changes on this pull request. So, going to let Taylor to fully validate it.