Implement FieldMask Well-Known type

[alexeyten]

https://developers.google.com/protocol-buffers/docs/reference/google.protobuf#fieldmask

This should serialize to JSON as a comma-separated string.
And probably in JS this should be represented as an array of string. E.g. this

syntax = "proto3";

package dummy;

import "google/protobuf/field_mask.proto";

message GetFields {
  uint32 id = 1;
  google.protobuf.FieldMask fields = 2;
}

should be represented as an object { id: 1, fields: ['foo', 'bar', 'baz.quux'] }.
Now it is represented as { id: 1, fields: { paths: ['foo', 'bar', 'baz.quux'] }.

[stephenh]

Closing this out thanks to @buyology 's PR; thanks Robin!

(@alexeyten fwiw I'm open to a follow up issue to represent FieldMask as a string[] directly in the message as well; currently it's a string[] in the JSON, but still a FieldMask message type.)