feat(conn): Generic over executor and update docs

[sjcobb2022]

Updates to the client-conn-layers to make it generic as needed and add docs.

Also has a minor change for pool Map docs that was trying to reference a private type.

[seanmonstar]

Oh, thanks! Let me just rebase my own branch on master to clear up the unrelated parts.

[sjcobb2022]

Do you think it would be worth mentioning that HttpXLayer or httpX() are generic over the body type that one wants? I left it out because I thought it was slightly intuitive but may be worth mentioning