authn: add support for custom callback url

[simaotwx]

Related: greenpau/caddy-security#64

[greenpau]

@simaotwx , thank you! 👍 Please add b.Config.JsCallbackURL too and handle the logic there too.

Additionally, how do you deal with

go-authcrunch/pkg/authn/handle_external_login.go

Lines 30 to 33 in 27766dc

	if strings.Contains(r.URL.Path, "-js-callback") {
	// Intercept callback with Javascript.
	return p.handleJavascriptCallbackIntercept(ctx, w, r)
	}

go-authcrunch/pkg/authn/handle_external_login.go

Lines 118 to 124 in 27766dc

	if (i < 0) {
	redirectURL = redirectURL.replace('authorization-code-js-callback', 'authorization-code-callback');
	window.location = redirectURL;
	} else {
	ridirectURI = redirectURL.slice(0, i).replace('authorization-code-js-callback', 'authorization-code-callback');
	window.location = redirectURI + "?" + redirectURL.slice(i+1);
	}

[greenpau]

@simaotwx , please use switch instead of if/else.

[greenpau]

@simaotwx , please replace CallbackUrl with CallbackURL.

[greenpau]

@simaotwx , please use switch instead of if/else.

[greenpau]

@simaotwx , also, please add your CLA consent to this commit.

[greenpau]

@simaotwx , additionally, please add tests.

[steverweber]

was this dropped or merged?

[simaotwx]

@steverweber dropped. Haven't found the time to do all the things mentioned. Nevertheless, the commit works for my use case so you can pick it if you feel like you need it, too.