Add support for custom asset domains

[michaela-bielikova]

Motivation

Customers want to use their own domains in asset links instead of the default Kontent asset URL. This way the links on their webpage will look more trustworthy, and since the custom links will be visible in search engine results, it also helps SEO optimization.
We will implement the support for custom asset domains in the Kontent app BE and deliver, but for the whole solution to work, we also need the support in SDK.

Proposed solution

We would like to add a new item to SDK configuration - custom asset domain. The customer would be able to set the domain. When the customer gets the item/variant that contains an asset (in Asset element or Rich text element), the custom domain will be used in the link instead of the default Kontent domain. The rest of the URL will stay the same.
Example: https://test15641652.assets.com/3727c9f0-9ea0-0068-2d4f-e6cdb4acff9d/1e040cde-e4be-4dac-a006-d406e40abf56/cat3.jpg instead of https://qa-assets-eu-01.freetls.fastly.net/3727c9f0-9ea0-0068-2d4f-e6cdb4acff9d/1e040cde-e4be-4dac-a006-d406e40abf56/cat3.jpg

Additional context

Release date: the BE part of the solution is almost ready, the last changes will probably be released 15.12.2021. The implementation of SDK does not have to wait for the release, it just cannot be released sooner than the BE part.