[BUG][GO] go api_* files fail to import time module

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? 4.3.1, 5.0.0 Beta in both go and go-experiemental modes
• Have you search for related issues/PRs? Yes .I filed similar in swagger-codegen
• What's the actual output vs expected output? actual api*.go files lack "time" module import while expected out has it
• [Optional] Bounty to sponsor the fix (example)

Description

go client api_* files do not import time module when the generated code contains references to time.Time.

This results in go compilation failure

openapi-generator version

I tested both 4.3.1 and 5.0.0 Beta in both go and go-experiemental modes

OpenAPI declaration file content or url

dt_test1.yml

openapi: 3.0.0
info:
  title: DateTime DataObject Test
  version: version.version0

paths:
  /test_fail:
    post:
      operationId: DateTimeImportFail
      responses:
        '200':
          description: test
          content:
            "application/json":
              schema:
                type: string
                format: date-time

Command line used for generation

I used 4 command lines to test all combinations

4.3.1 Go - java -jar openapi-generator-cli.jar generate -g go -i dt_test1.yml -o og_dt/go
4.3.1 Go experimental - java -jar openapi-generator-cli.jar generate -g go-experimental -i dt_test1.yml -o og_dt/goe
5 Beta go - java -jar openapi-generator-cli-5.0.0-beta.jar generate -g go -i dt_test1.yml -o og5_dt/go
5 Beta Go Experimental - java -jar openapi-generator-cli-5.0.0-beta.jar generate -g go-experimental -i dt_test1.yml -o og5_dt/goe

Steps to reproduce

Define an operation returning date-time string value and generate go bindings. The results will refer time.Time type and not import the time module

For example 5.0.0 Beta go experiemental produces with the above api_default.go like the following

....
import (
	_context "context"
	_ioutil "io/ioutil"
	_nethttp "net/http"
	_neturl "net/url"
)
....
/*
Execute executes the request
 @return time.Time
*/
func (r apiDateTimeImportFailRequest) Execute() (time.Time, *_nethttp.Response, error) {

Related issues/PRs

No

Suggest a fix

Import the time module when time.Time is used in api_*.go files

Start with dt_test1.yml and run the listed openapi-generator CLI command using the Go generator to reproduce the generated api_default.go output. Inspect how imports are selected for the generated time.Time return type; done means the generated api_*.go files import time and compile successfully.