Add 'web_scheme' as a primary attribute to Domain

[Szasza]

Issue

In v4.8.2 the web_scheme domain attribute is only supported as a create domain operation's option. As per the documentation however it is a primary attribute of the Domain entity: https://documentation.mailgun.com/en/latest/api-domains.html#domains

Also the current implementation only supports web_scheme in case of a domain creation, but not for retrieval or update.

Desired outcome

Having web_scheme supported as a primary attribute of the Domain entity.

[Szasza]

@thrawn01 @teros0 I am happy to work on this one if you are ok with the approach.

[teros0]

Sounds like a great addition to me.
@thrawn01 what do you think?

[thrawn01]

This would be a great addition!