Cosmos V4: Reduce initialization confusion

[bartelink]

re #197 This addresses a longstanding mess in Cosmos.fs wrt how initialization is managed
@ylibrach this has placeholder names that would benefit from some pairing if you're up for it ;)

[ylibrach]

@bartelink from what I can tell, this removes the ability for the consumer to provide their own implementation of ContainerGateway, which was the whole point of the client abstractions work. Unless I'm not seeing something?

[bartelink]

Apologies; Its possible that it's not wired through, and I agree its a pretty big gap
(I was parking that final wiring work pending your high level review at the time (and it was definitely dark outside ;) ))

[bartelink]

The above exposes a createGateway argument on the Equinox.Cosmos.Client - can you see if it looks right please? @ylibrach
Feel free to merge and/or post follow-up comments or commits as you see fit...

[bartelink]

More stuff will likely happen to more closely align with https://azure.github.io/azure-sdk/dotnet_introduction.html before a V3 beta...

[bartelink]

Merging as agreed out of band with @ylibrach - some naming and signature changes will follow as independent PRs