Allow self hosted customers to specify Datadog widget host

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

Currently, our Datadog UI extensions assume that the user's environment and project are hosted on app.flagsmith.com. This is not usable for self hosted options.

**Describe the solution you'd like**

Allow the Datadog widget host to be configurable via an option

- When a widget loads, prior to checking authentication, check the domain vs the configured option, if it doesn't match then redirect.
- When the correct host is identified, keep the same setup process as now.

**Describe alternatives you've considered**

Sharing UI extension config with self hosted users.

**Additional context**

Initial message

```
Hello, is it possible to integrate Flagsmith with Datadog when using self hosted? When trying to connect with the Flagsmith widgets in a DD dashboard, I am redirected to https://app.flagsmith.com/ and don't see an option to change to the url I host flagsmith at.
```