Add new SPECIAL_EVENT Cause to GTFS-realtime Service Alerts

### Describe the problem

Many producers find it difficult to select a Cause from the current list in certain common situations, and we've heard complaints from customers about this.

I'm proposing to add SPECIAL_EVENT to address this. Capturing alert causes also provides more structured data, which could be used to display Alerts in certain ways on agency websites and potentially in apps.

### Use cases

SPECIAL_EVENT would be the appropriate Cause to select in the following common scenarios faced by many agencies:

- Parades and festivals
- Marathons, walks and runs that shut down streets
- Concerts and other performances
- Farmers markets

### Proposed solution

Add SPECIAL_EVENT to the Cause list as option 13 as follows:

// Cause of this alert. If cause_detail is included, then Cause must also be included.
enum Cause {
UNKNOWN_CAUSE = 1;
OTHER_CAUSE = 2; // Not machine-representable.
TECHNICAL_PROBLEM = 3;
STRIKE = 4; // Public transit agency employees stopped working.
DEMONSTRATION = 5; // People are blocking the streets.
ACCIDENT = 6;
HOLIDAY = 7;
WEATHER = 8;
MAINTENANCE = 9;
CONSTRUCTION = 10;
POLICE_ACTIVITY = 11;
MEDICAL_EMERGENCY = 12;
**SPECIAL_EVENT = 13; // A special one-time or recurring event such as a parade, festival, performance, farmers market, or sporting event.**
}

### Additional information

There has been some discussion about this in the MobilityData Slack. @stevenmwhite @jfabi @doconnoronca and Logan Nash have chimed in, and it seems consensus for this change.

@gcamp and @bdferris-v2 could you please weigh in from Transit and Google's standpoint? How do you use Cause values currently, if at all?

I look forward to hearing any feedback about this idea, particularly in the following areas:

- What other use cases do you see for this new Cause?
- Are there other Cause values we could add/revise (as minimally as possible) such that this list encompasses at least 90% of the most common actual scenarios, and at least 90% of the most common scenarios currently leading to use of Other_Cause?
- Are there any unintended consequences for apps, websites or other Alerts consumers?