Add message option to serialize as well known JSON type

[martindam]

This is a suggested implementation of #738

With this diff it is possible to mark that a message should be JSON marshalled as a well known type

Without jsonwkt option:

message UUID {
	string value = 1;
}
message MyType {
	UUID user_uuid = 1;
}

MyType would be serialized to JSON as:

{
	"user_uuid": {
		"value": "8dfab8c2-7916-477f-bbd6-c9fc00dc4158"
	}
}

With jsonwkt option

message UUID {
	option (gogoproto.json_well_known_type) = "StringValue";
	string value = 1;
}
message MyType {
	UUID user_uuid = 1;
}

MyType would be serialized to JSON as:

{
	"user_uuid": "8dfab8c2-7916-477f-bbd6-c9fc00dc4158"
}