Allow incremental addition of HTTP headers to generated Go clients

[nyergler]

Feature Description

We have two different auth mechanisms for our API: one uses a bearer token and one uses basic auth.

I was confused recently as to why some requests weren't coming through with the HTTP User-Agent I was configuring. Then I realized that it was because the option.WithHTTPHeader stomps on previously set headers.

For example, constructing a client with the following:

    return ptclient.NewClient(
        ptoption.WithBaseURL(deploymentURL),
        ptoption.WithHTTPHeader(http.Header{
            "User-Agent": []string{UserAgent},
        }),
        ptoption.WithHTTPHeader(http.Header{
            "Authorization": []string{"Basic " + basicAuth(p.DeploymentKey, "")},
        }),
    )

Replaces the user agent with the authorization. (This is slightly more obvious in this example; in my case I had a function that was applying the "base" options ---use, user-agent -- and appending the passed in auth option(s)).

I think it'd be clearer to if WithHTTPHeader was additive, with a option.WithHTTPHeaders (plural) that does the current behavior. Since changing the semantics may be tricky, perhaps an AddHTTPHeader is a reasonable name for an additive case.

Proposed Generated Code (Optional)

// HTTPHeaderOption implements the RequestOption interface.
type HTTPHeaderOption struct {
	HTTPHeader http.Header
	Replace bool
}

func (h *HTTPHeaderOption) applyRequestOptions(opts *RequestOptions) {
	if h.Replace {
		opts.HTTPHeader = h.HTTPHeader
	} else {
		opts.HTTPHeader = append(opts.HTTPHeader, h.HTTPHeader...)
	}
}

// WithHTTPHeader replaces the http.Headers for the request.
func WithHTTPHeader(httpHeader http.Header) *core.HTTPHeaderOption {
	return &core.HTTPHeaderOption{
		// Clone the headers so they can't be modified after the option call.
		HTTPHeader: httpHeader.Clone(),
	}
}

// AddHTTPHeader adds the given http.Headers to the request.
func AddHTTPHeader(httpHeader http.Header) *core.HTTPHeaderOption {
	return &core.HTTPHeaderOption{
		// Clone the headers so they can't be modified after the option call.
		HTTPHeader: httpHeader.Clone(),
		Replace:true,
	}
}