Encode structures with all non-Optional member shapes using fixed-length CBOR maps

[david-perez]

CborSerializerGenerator serializes structure shapes using indefinite-length CBOR maps. However, when all the member shapes are non-Optional, we know ahead of time the fixed length, and we could use a definite-length encoding for added efficiency.

The only implementation difficulty here is that we serialize an added field, __type, as a customization, in AddTypeFieldToServerErrorsCborCustomization.kt, when serializing error responses in the server, so depending on whether the customization was added or not, we need to encode an extra field. A possibility could be to simply gate the __type-encoding behavior behind a constructor flag in CborSerializerGenerator as opposed to using a customization.

It'd be interesting to tackle this after we have proper benchmarks (#214) to determine any performance impact.