Need a way to better express named type derivations

[cmars]

Need a way to define a type derivation that conveys a constraint or format.

Don't want to use custom format: with type: string because this requires too much coordination among systems that will need to interpret the formats in the same way.

pattern: is better, and precisely defines constraints, but is fugly in generated documentation.

What we want is pattern: with a "type derivation name" for the constraint that can be displayed in documentation (this is a URL / version string / cursor / etc) in terms a user will understand.

Will likely require a bit of OpenAPI extension to express.