Don't reject arbitrary `--type` values

[mikix]

When passing in a --type that we don't normally support, we should probably allow it. Right now we reject any that aren't in our default set.

• What to do about --since-mode=created? That requires special per-resource knowledge.

[mikix]

Complications: grabbing linked resources like Practitioner, etc. Would need a more dynamic approach for that if we allowed this.