TEL-730: Update various SIP validation methods to return error slices.

[alexfish8]

Motivation: we'd like like to get a better sense as to which clients are sending invalid REFER messages (see TEL-730). In order to do so, we're updating various SIP validation methods to return slices of errors. Some of these errors might be "soft errors" (i.e., encountering them shouldn't cause the surrounding request to fail). Previously, these soft errors were could be iterated over via a global SoftFailureNotifier. The global nature of this notifier made it cumbersome to log these soft errors with request-scoped loggers. This PR updates the validation methods to include these soft errors in their return values so that callers may log them as they please.

A few additional notes:

• In general, I've tried to avoid updating err return values to []error unless I needed to do so.
• Additionally, so as to avoid introducing unintended behavior changes, I've tried to keep all return paths consistent in functions where I've updated the return type signature (i.e., if, previously, an err was returned eagerly, now, we'd eagerly return an error slice consisting of just that error). In the future, we might consider updating some of these methods to return a collection of all validation failures, but, I'd like to avoid introducing a lot of changes all at once.
• Add a few helper functions to extract "hard errors" from these error slices (thanks @alexlivekit for the suggestion on LogSoftErrors).

TODO: Maintain backwards compatibility so that we don't have to do a major version bump.

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[alexfish8]

Closed in favor of #1667.