feat: add SetBodyDecoder for custom JSON body decoding

Add a BodyDecoderFunc type and SetBodyDecoder method on HTTPAdapter
that lets consumers customize how JSON request bodies are decoded.
The default remains runtime.DecodeJSONBody.

Source changes are limited to two files:
- pkg/runtime/json.go — BodyDecoderFunc type
- pkg/codegen/templates/handler/adapter.tmpl — field, default, setter

Use cases:
- Pre-process request bodies before decoding (e.g. strip nulls)
- Use a lenient decoder that tolerates schema mismatches
- Enrich decode errors with application-specific context

Author: joncanning-rvu