[TypeScript] Use interfaces for fetch request parameters

Description

Request parameters should be in interfaces, same as for general object definitions.
I am using http://swagger.io/specification/#parametersDefinitionsObject and would like to be able to use the typed parameter definitions.

Swagger-codegen version

2.2.1

Swagger declaration file content or url

swagger: '2.0'
paths:
  /foo
    get:
      parameters:
        - $ref: '#/parameters/skipParam'
        - $ref: '#/parameters/limitParam'
skipParam:
  name: skip
  in: query
  description: number of items to skip
  required: true
  type: integer
  format: int32
limitParam:
  name: limit
  in: query
  description: max records to return
  required: true
  type: integer
  format: int32

Should produce

export interface FooApiParams {
  /** number of items to skip */
  skip: number;
  /** max records to return */
  query: number;
}
export class FooApi extends BaseApi {
  fooGet(params: FooApiParams) {
    ...
  }
}

instead of what it currently produces:

export class FooApi extends BaseApi {
  fooGet(params: { skip: number, limit: number }) {
    ...
  }
}

Command line used for generation

java -jar ${jarPath} generate -i swagger.yaml -l typescript-fetch -o ${dir}

Steps to reproduce

Generate any typescript-fetch api where the swagger file has a parameters object with definitions used in some path.

Related issues

Suggest a Fix

Fixing shouldn't be hard; all the information is already available, this line and the similar lines in the same file could simply be changed to something like

    {{nickname}}({{#hasParams}}params: {{nickname}}Params }, {{/hasParams}}options?: any): FetchArgs {

where {{nickname}}Params is defined alongside the regular object definitions with something like

{{#operations}}
{{#operation}}
export interface {{nickname}}Params {
{{#allParams}}
    {{paramName}}"{{^required}}?{{/required}}: {{{dataType}}};
{{/allParams}}
{{/operation}}
{{/operations}}

I don't actually know mustache so the above is just a guess based on the existing template!

Start with modules/swagger-codegen/src/main/resources/TypeScript-Fetch/api.mustache, especially the referenced operation and object-definition sections. Generate a typescript-fetch API from the supplied Swagger parameters example using the documented command, then verify that the output defines a per-operation params interface and uses it in the API method instead of an inline object type.