FIX/ENH: fix ioc-deploy auth, provide ssh-agent-helper

[ZLLentz]

Description

• [x] Switch the http checks in ioc-deploy to git clone checks using ssh auth.
• [x] Provide ssh-agent-helper for using the ssh-agent based on https://github.com/pcdshub/twincat-bsd-ansible/blob/master/scripts/ssh_agent_helper.sh
• [x] Warn users that they might want to try the ssh-agent-helper

Motivation and Context

• ioc-deploy currently only works on public repos because it receives 404s from http checks of internal and private repos for logged out users. If we instead use the user's shell's existing ssh authentication, we don't need to pass passwords around, distribute PATs, etc.
• I'd like to provide some ssh-agent helpers so that people only have to type in their ssh key one time. It's surprisingly nuanced to set these up, it's best practice to put time limits on your ssh-adds and make sure to clean up the agent on shell exit. The result is pretty concise but those are hard-fought lines of bash script...

How Has This Been Tested?

Interactively only

Where Has This Been Documented?

Only here, so far

[ZLLentz]

I chatted with Robert and our consensus is that this probably shouldn't set up SSH agents, but it should let you know that your agent isn't set up and suggest actions to fix this. There should also be some helpers for creating SSH keys.

[ZLLentz]

I fixed the stdout/stdin typo and greatly expanded the comments in ssh-agent-helper so it can be useful as a primer on how the agent setup works.

[ZLLentz]

One more to-do: I need to add ssh-agent-helper to the README

[janeliu-slac]

Sorry, I didn't have much to contribute. Read through the file changes and brushing up on bash scripting.