Inventory-based SSH proxy/bastion host support, ssh_args + docs

Supersedes #9122 

First-class inventory var support for proxy/bastion/jump hosts seems to be increasingly requested. I've tried to make it reasonably comprehensive and well-documented, though I intentionally omitted proxy_password support (had some trouble stacking the file descriptors for the two-level-deep sshpass).

(CC @sivel)

Yeah, I considered using netcat as a lowest-common-denominator approach. However, I saw some complaints around the openssh community about instability with various versions of netcat that the -W mode attempts to address (don't have a reference to share off the top of my head- it was several months ago). If a bastion host can't be upgraded to a reasonably recent version of openssh, the ssh_args escape hatch is still there- I think that beats the alternatives of:
- forcing everyone to use a potentially unstable approach that has another dependency (netcat)
- adding yet another mode-changing variable (ansible_ssh_proxy_mode=netcat or some such blah)
- version-sniffing ssh (rife with issues)