feat(domains_v1): add support for new UDM endpoints

[Integralist]

Here are the UDM endpoints being added to go-fastly:
https://www.fastly.com/documentation/reference/api/domains/domain/

[tundal45]

Lots of questions all of which are for my edification and non-blockers. I have one nit which is also not a blocker.

[kpfleming]

I've left a number of suggestions, and requested that plural operation names be changed to singular when they operate on a single domain.

Now, a more radical suggestion: I'm really not a fan of these operations being labeled "DomainsV1" :-) I've recently added the first operations in go-fastly which are not in the fastly namespace directly (they are below it). A similar thing could be done here, locating these operations at fastly/domains/v1, and then removing DomainsV1 from all the structure and function names to simplify them.

I think this will be easier for users to understand, but it's definitely not mandatory. Moving things out of fastly also helps with testing as you can't accidentally rely on things that are unexported (which I ran into while working on fastly/products.

[Integralist]

@kpfleming I've made the changes you've suggested.

I'm just going through the more radical change you suggested, to see if it's something I can turn-around in a reasonable time frame (as we're bit strained for time as far as getting this work done and so now might not be the time to do that change about). I'll let you know later how I get on.

[kpfleming]

Just a few stragglers in comments, otherwise this is fine.

[Integralist]

@kpfleming the design is looking a lot nicer now 👍🏻

If you could give this another review please. Thanks!

[cee-dub]

Looks clean in the v1 directory. 👍

[kpfleming]

One minor change, otherwise this is good to go. It looks great!