feat: Initial support for Fares v2 validation.

[bdferris-v2]

Per issue #1201, adds validation for files and fields added for Fares v2 base spec changes.

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[bdferris-v2]

Initial draft, mostly to see what happens when validating a few existing feeds.

[bdferris-v2]

Now that #1248 has been committed and integrated, this PR is ready for official review.

[maximearmstrong]

fare_transfer_rule_forbidden_duration_limit

• spec mention in fare_transfer_rules.duration_limit_type: Forbidden if fare_transfer_rules.duration_limit is empty.
• pseudo code: If [fare_transfer_rules.duration_limit == '' and fare_transfer_rules.duration_limit_type != ''], trigger the notice

@isabelle-dr I think this has been already added with the FareTransferRuleDurationLimitWithoutTypeNotice here. Could you confirm this is what you were looking for?

[bdferris-v2]

I believe @isabelle-dr was correct that I didn't have a validation check for when duration_limit_type was specified but duration_limit was not. I've added that now.

I believe all validation rules have documentation in RULES.md at this point. Let me know if you see things differently.

As for fare_rules.txt and fare_attributes.txt, those seem to be clarifications of the Fares V1 spec. I'm happy to add support for that, but I'd argue for splitting it out into a separate PR to keep things (relatively) focused. Thoughts?