feat: Add ParamsStruct in middleware.Request

[akkuman]

I wish to support custom validate（like https://github.com/go-ozzo/ozzo-validation）, but it only support validate on body currently

example middleware:

package middleware

import "github.com/ogen-go/ogen/middleware"

type validatable interface {
	Validate() error
}

// Validator 一个适用于 ogen 的校验中间件
func Validator() middleware.Middleware {
	return func(req middleware.Request, next middleware.Next) (middleware.Response, error) {
		if params, ok := req.ParamsStruct.(validatable); ok {
			err := params.Validate()
			if err != nil {
				return middleware.Response{}, err
			}
		}
		if body, ok := req.Body.(validatable); ok {
			err := body.Validate()
			if err != nil {
				return middleware.Response{}, err
			}
		}
		return next(req)
	}
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.87%. Comparing base (b1ddda2) to head (c94a623).
Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1204   +/-   ##
=======================================
  Coverage   73.87%   73.87%           
=======================================
  Files         189      189           
  Lines       12772    12772           
=======================================
  Hits         9435     9435           
  Misses       2797     2797           
  Partials      540      540           

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