feat: support appcues integration custom domain option

[zhukevgeniy]

What does this PR do? This PR adds a custom domain (also known as CNAME) option for appcues integration. Currently, the only way to use a custom domain for appcues is to integrate directly with their SDK, So by providing this custom option, a user will still be able to use appcues as a segment integration.

Link: https://docs.appcues.com/dev-installing-appcues/host-appcues-sdk-under-your-own-domain

Are there breaking changes in this PR? NO

Testing No special tests are needed

Any background context you want to provide?

Is there parity with the server-side/android/iOS integration components (if applicable)?

Does this require a new integration setting? If so, please explain how the new setting works

Links to helpful docs and other external resources