Support for google.protobuf.FieldMask well known type

[mattnathan]

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).

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.