OpenAPI V3 schema default type should support any

I noticed in the code generator for Schema.default, the only acceptable types are bool, string, float, and int. https://github.com/google/gnostic/blob/master/generate-gnostic/generate-compiler.go#L233-L257

The OpenAPI 3.0 schema defines that the default value can be anything (https://github.com/OAI/OpenAPI-Specification/blob/main/schemas/v3.0/schema.json#L513-L514) and does not specify this oneOf clause present in gnostic.

We should be able to support Any for the default

cc @timburks