Add Echo body cache middleware - #6

Summary

• add an opt-in BodyCache middleware that stores the original request body with a configurable max size
• add a CachedBinder wrapper that restores c.Request().Body after c.Bind() so downstream handlers can read it again
• cover JSON, XML, form-encoded, empty body, oversized body, and RawBody copy behavior

/claim #1

Testing

• Not run locally: Go is not installed in this environment.