Backport lib/utils/prompt improvements to [v9]

[codingllama]

prompt.ContextReader, along with various parts of lib/utils/prompt, where re-written in master so we can alleviate input swallowing issues.

For various reasons I didn't backport all changes to v9, but the less-eager input-swallowing loop is now what stands between us and issue #13021.

This isn't a backport of a specific PR, but instead a port of the entire lib/utils/prompt package, as the PRs that touch the package unfortunately do more than we want to backport. The interfaces are still compatible and I did test various tsh login and tsh ssh scenarios.

I've thrown in #13382 for good measure, as well.

Closes #13021.

[codingllama]

The initial commit is equivalent to:

rm lib/utils/prompt/*
git checkout master -- lib/utils/prompt/

tsh scenarios, including <ctrl+c> in pretty much every step. Showing so we can see the prompts.

$ ./tsh login --proxy=zarquon --user=llama
> Enter password for Teleport user llama:
> Tap any security key or enter a code from a OTP device:
> (ok)

$ ./tsh ssh --proxy=zarquon --user=llama llama@zarquon
> Tap any security key
> (ok, no input swallowed)

$ ./tsh ssh --proxy=zarquon --user=llama --mfa-mode=otp llama@zarquon
> Enter an OTP code from a device: <otp>
> (ok, no input swallowed)

[codingllama]

Thanks for the quick reviews, folks.