Support application/x-www-form-urlencoded post body (form data)

[fbjorn]

Currently the library is built for JSON APIs. It can't handle other types of data, for example application/x-www-form-urlencoded.

We need to consider adding support of it.

Example of the spec:

export interface paths {
  "/token": {
    /**
     * Login 
     * @description Login endpoint.
     */
    post: operations["login_token_post"];
  };
  // ...
}

export interface operations {
  login_token_post: {
    requestBody: {
      content: {
        "application/x-www-form-urlencoded": components["schemas"]["Body_login_token_post"];
      };
    };
    responses: {
      /** @description Successful Response */
      200: {
        content: {
          "application/json": {
            [key: string]: string | undefined;
          };
        };
      };
      /** @description Validation Error */
      422: {
        content: {
          "application/json": components["schemas"]["HTTPValidationError"];
        };
      };
    };
  };
  // ...
}

export interface components {
  schemas: {
    /** Body_login_token_post */
    Body_login_token_post: {
      /** Grant Type */
      grant_type?: string | null;
      /** Username */
      username: string;
      /** Password */
      password: string;
      /**
       * Scope 
       * @default
       */
      scope?: string;
      /** Client Id */
      client_id?: string | null;
      /** Client Secret */
      client_secret?: string | null;
    };
    // ...
  }
}