Default to creating agent iframe on widget create

[greenberga]

There are a few things happening in this pull request.

1. No more baseURL on FriendlyCaptchaSDK

Can cause problems in the (rare) event of having widgets with different API endpoints on a single page. Instead, each widget is responsible for "setting up" an apiURL.

2. opts.startAgent, if truthy, can be used to pre-load the agent iframe at SDK-construction.

In that case, it will configure the agent iframe using the endpoint passed as an option to the SDK constructor, falling back to the search algorithm outlined in options.ts, and finally defaulting to the global endpoint.

3. In the default case, the agent iframe is now created upon widget creation

The widget also takes care to add the correct origin to the CommunicationBus. I think the original timeout errors might've been caused by a CommunicationBus configured with the global endpoint in combination with an eu widget. I.e., the problem wasn't that there were two iframes, it's that there was only a global origin in the CommunicationBus, and the EU widget couldn't use it.

Fixes https://github.com/FriendlyCaptcha/friendly-captcha/issues/1889

[greenberga]

By the way, this passes all the tests when I run go run main.go autotest except csp. But csp seems to be failing on main too, so I'm not too worried about that.

[gzuidhof]

I'm happy to lose this 👍