codegen: Support declaration deduplication (WIP)

[hughsimpson]

Would address #4719

The idea for this was prompted by a somewhat idiosyncratic methodology for versioning that I'm using, where each versioned api is fully declared as a separate openapi. I don't want the compilation overhead of re-generating schemas and serdes for each version where there are no changes to a particular schema, and I do want to be able to reuse security logic without fiddling about with conversions between otherwise-identical types in different namespaces.

I genuinely think this could also be useful for other use-cases, such as sharing common types (e.g. errors) between different APIs.

Currently done:

• model dedup
• tapir schema dedup
• security schema dedup
• annotation dedup
• serde dedup (kinda still in progress, has bugs)
• parse openapi dir structures (probably has bugs, not really tested this yet)

To do:

• validator dedup
• endpoint dedup
• more than one 'inherited' openapi
• lotsa tests