Fix control keys handling

[jbbarth]

Issue #: #29

Description of changes: this change switches the terminal in "raw" mode so that it handles Ctrl+key sequences better.

• IMPORTANT: I have very little experience with Go, and very little experience with low level terminal applications. I'm not sure at all this is the right fix.
• I suspect some of the stty "manual" manipulations in the codebase could be replaced by what the "term" package does, but again, I know nothing ; I tried removing them and the behaviour doesn't change
• ⚠️ the fix seems to add a new blank line after the end of the session, it's ugly but it doesn't seem to have a real effect

How I tested:

# build (I'm on a MacOS M2 so darwin-arm64)
rm -rf vendor/src/github.com/aws/session-manager-plugin/session-manager-plugin && gofmt -w src && make clean release && cp bin/darwin_arm64_plugin/session-manager-plugin ~/bin

# test before the modification ("mainline")
# => behaviour similar to homebrew installed version, Ctrl+K crashes, Ctrl+O doesn't work

# test after the modification (this PR)
aws ssm start-session --target <instance>
# basic commands
ipython
# => ctrl+O => works
# ctrl+K => doesn't kill the session
# ctrl+C, D => still working
# ctrl+J => works at first sight in the shell (new shell prompt)

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jbbarth]

Auto-review: this could be moved to Stop() too, I don't have a strong preference. The behaviour doesn't change when I do it (we then need to add a new field originalTermState term.State on the ShellSession struct).

[jbbarth]

Auto-review: the project doesn't seem to use "go.mod", so I blindly cloned the repository:

git clone https://github.com/golang/term.git vendor/src/golang.org/x/term

... and removed the .git directory there.

[AndrewFarley]

Note: This fix works great for me and makes the "JOE" CLI editor (yes, old I know) common escape sequences to work properly and not cause random issues or shell ejection. I can't speak for the code quality, but I thumbs-up this from a functionality point of view.

[jlebonzec]

Any chance to see this merged into the main SSM code?

[AndrewFarley]

The longer we wait, the more someone has to go fix conflicts. Please figure this out and merge this. I'm using this locally and it fixes all my issues and complaints with SSM.

[ghthor]

Gonna get to 2 years, classic AWS

[ghthor]

@Yangtao-Hua @ziwangj Yall need some help maintaining, reviewing and accepting patches from the community? Happy to help.