Validate filtering client queries on the client

[alexander-yevsyukov]

Suppose I want to subscribe to messages of a certain type filtering by the value of a field. Suppose I made a mistake in the name of the field. The mistake would result in server-side error because we don't validate the reference when the subscription is posted. This is:

1. too late;
2. hard to debug.

We need to have client-side validation of field references before the request is submitted to the server.

[alexander-yevsyukov]

It can be made via a mix-in interface. We know the name of the type, we can obtain the descriptor of the message. Get the field reference and see if the field is there.