chore: check stops.zone_id presence

[lionel-nj]

closes #237

Summary:

This PR provides support to implement #237

Expected behavior:

New notice:

• StopWithoutZoneIdNotice (severity level = WARNING - to be upgraded to ERROR in the future).

Per GTFS specification, stops.zone_id should be provided if fare information is provided using fare_rules.txt - if this requirement is not met, a notice should be emitted.

Note that this rule does not apply to records from stops.txt that represent a stop or en entrance (i.e. stops.location_type = 1 or 2) - in this case, no notice should be emitted.

Gtfs specification reference:

| zone_id | ID | Conditionally Required | Identifies the fare zone for a stop. If this record represents a station or station entrance, the zone_id is ignored.

Conditionally Required:
- Required if providing fare information using fare_rules.txt
- Optional otherwise.|

from https://github.com/google/transit/blob/master/gtfs/spec/en/reference.md#stopstxt

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• [ ] Include screenshot(s) showing how this pull request works and fixes the issue(s)

[maximearmstrong]

LGTM :)

[aababilov]

This Java code was not formatted, so an autoformat commit was submitted automatically. As we discussed before, such commits have a disadvantage - they hide the actual ownership of changed lines.

1. Please format the Java code before submitting. You can use google-java-format -i $(find . -name '*.java')
2. Please add an automatic test that a pull request contains properly formatted code. Unformatted code may not be submitted.

[lionel-nj]

This Java code was not formatted, so an autoformat commit was submitted automatically. As we discussed before, such commits have a disadvantage - they hide the actual ownership of changed lines.

Indeed, we have a workflow to check said formatting. After verification it used to only execute on push on master.
#953 fixes that: the workflow is executed on pull requests and fails if PR's code is not formatted properly.

[aababilov]

This return exits the whole validate function and interrupts validation of the rest of the file. Same for the return below.

[lionel-nj]

The logic here has been modified in #972 where validate is as follows:

gtfs-validator/main/src/main/java/org/mobilitydata/gtfsvalidator/validator/StopZoneIdValidator.java

Line 48 in 18c7921

public void validate(NoticeContainer noticeContainer) {