[REQ][Golang][Client] Add `*http.Client` Accessor

[grokify]

Is your feature request related to a problem? Please describe.

It can be useful to access the *http.Client directly when certain APIs cannot be currently supported by OpenAPI Generator.

This is currently inaccessible in *APIClient.cfg.HTTPClient

Describe the solution you'd like

An accessor to retrieve the *http.Client for usage.

Describe alternatives you've considered

Option 1

I currently add the following to client.go in all my generated clients:

// HTTPClient returns the HTTP client for direct use.
func (apiClient *APIClient) HTTPClient() *http.Client {
	return apiClient.cfg.HTTPClient
}

Option 2

Add a HTTPClient accessor to APIClient so it does not have to be first retrieved.

For example:

type APIClient struct {
	cfg        *Configuration
        HTTPClient *http.Client // proposed
...

func NewAPIClient(cfg *Configuration) *APIClient {
	if cfg.HTTPClient == nil {
		cfg.HTTPClient = http.DefaultClient
	}

	c := &APIClient{}
	c.cfg = cfg
        c.HTTPClient = cfg.HTTPClient // proposed
...

Additional context

None.