Support for google.protobuf.FieldMask well known type

Consider supporting `FieldMask` in a more advanced way by allowing you to select the properties of a message that should be included.

Some nice to have options and things to consider:

1. Flatten the `paths` property to be top level instead of having it nest
2. The ability to switch back to the `string[]` approach for any edge cases
3. The ability to specify which message/property the fields are relative to
4. Maybe a _set from request_ button that creates a FieldMask paths from all the included fields you've already filled out in your request

**Example**
My API has a pattern where resource update methods look like `UpdateFoo(UpdateFooRequest)`.

```protobuf
message UpdateFooRequest {
Foo foo = 1;
// fields relative to foo
google.protobuf.FieldMask update_mask = 2;
// ... other update metadata
}
```

Which allows for updating specific fields in the resource (`Foo` in this case), I thought more advanced support for FieldMask would help out.