fix(transfers): min_transfer_time is conditionally required for timed transfers

[rudiejd]

Describe the Problem

The wording of min_transfer_time vaguely indicates that it might be required for timed transfers (transfer_type = 2). This is the interpretation that some consumers, such as Google, accept, but the column is specified as optional in the spec so the requirement is not a formal one.

Proposed Solution

Make min_transfer_time conditionally required for timed transfers.

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Additional Information

MBTA had our feed rejected by Google because this requirement was not met. The requirement seems reasonable, but it is not specified strongly enough so as to be an unambiguous rule.

Checklist

• I have read and understood the GTFS Change process.
• I have read and understood the GTFS Guiding Principles.
• I understand the Role of Advocate and the Responsibilities attached to it.
• I have signed the Contributor License Agreement (CLA).
• I have ensured that this proposal is not covered by an existing Pull Request.

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