Phone number international format in GTFS

Currently GTFS doesn’t provide guidance for the format of phone numbers. There have been calls to include international extensions to allow for crossborder travellers to contact transit agencies seamlessly [[Example]](https://github.com/google/transit/pull/388#discussion_r1269171372).

There are several ways we can include international format in GTFS. Here are some options:
1. Create an extra field to allow for the international extension. This can cause some friction as not all formats work in the same way. Some examples:
- In NA, it would be +1 (XXX)-XXX-XXXX where no numbers are dropped
- In EU, it would be +XX (0)XX XXX XX XX where 0 is dropped when using +XX
2. Recommended usage of an international format with option to keep local format.
3. (Restricting the use of local format and allowing only the international format.) This would not be backward compatible.

Here is a [wikipedia](https://en.wikipedia.org/wiki/National_conventions_for_writing_telephone_numbers) for more information on the international format.