firestore: Add function to convert firestore protobuf document to a struct

Cloud Functions supports Firestore triggers and when a document is updated an event is send to the function with this format:

``` go
pb "google.golang.org/genproto/googleapis/firestore/v1"

// FirestoreEvent is the payload of a Firestore event.
type FirestoreEvent struct {
OldValue pb.Document `json:"oldValue"`
Value pb.Document `json:"value"`
UpdateMask struct {
FieldPaths []string `json:"fieldPaths"`
} `json:"updateMask"`
}
```
There is currently no way to convert `pb.Document` to a normal Go struct. [setFromProtoValue](https://github.com/googleapis/google-cloud-go/blob/61166ef305533a6c5b0b3275df2cb544d678339a/firestore/from_value.go#L27) could be exported to make it possible or a new helper function based on this [DataTo](https://github.com/googleapis/google-cloud-go/blob/27b5416a80e6a65ee9eebda55f06def51b9b7b88/firestore/document.go#L115) line.