Support for extensions within swagger:parameters

[cccRaim]

Summary

Related to #2751.

For i18n document, route almost can include more information, now I want the parameter has more information.

Example

Example
Input:

// swagger:parameters listPets
type FooParam struct {
	// Bar parameter in query
	// Required: true
	// Extensions:
	//   x-description-i18n:
	//     cn: 一个参数
	Bar string `query:"bar"`
}

Output:

paths:
  "/pets":
    get:
#       ...other info
      parameters:
        - description: Bar parameter in query
          in: query
          name: Bar
          required: true
          type: string
          x-description-i18n:
            cn: 一个参数