[Feature Request] String TimeZone validation

[dropwhile]

Feature description:
Since google.protobuf.TimeStamp is just seconds and nanos in UTC, a somewhat common solution to a lack of timezone information is to simply send a timezone along with it. Sending a numerical offset is sometimes fragile as offsets can change due to daylight savings or tz updates, so a tz string may be sent along with a timestamp instead.

It would be nice to have a means to validate that the timezone is valid, as per the cel timezone supported formats (eg. TimeZone = "UTC" | LongTZ , as I don't think FixedTZ makes as much sense in this context)

Problem it solves or use case:
Consider the following proto:

syntax = "proto3";
package some.thing.v1;

import "buf/validate/validate.proto";
import "google/protobuf/timestamp.proto";

message TimestampTZ {
  google.protobuf.Timestamp ts = 1;
  string tz = 2;
}

It would be nice to have something like:

  string tz = 2 [(buf.validate.field).string.timezone = true];