Add support for externally managed websocket event authorizers

[alanmun]

Externally managed authorizers (like cross account) for a Websocket $connect route seems to create an invalid permission, causing deployment to fail. I found out that this is the case because although the documentation seems to suggest that cross-account lambda authorizers are supported with websocket, they currently aren't. The code always creates permissions instead of skipping them as is done with http api.

This small PR adds code to not generate a warning anymore when you use "managedExternally" in "authorizers" since it exists now as well as code to detect that property and skip creating permissions. It also adds a documentation update to reflect that this is supported now more clearly.

I tried my best to follow CONTRIBUTING.md. If I did something wrong please let me know. I thought that this bug fix was simple enough to not warrant an issue. I consider it a bug fix because the documentation seems to imply that this is normally possible, but it wasn't:

I ran npm run prettify:update to only prettify the changed code, and npm run lint.

Oh, btw, I ran npm test and saw a bunch of output saying that certain tests were skipped due to not being able to create symlinks because no admin privileges. I actually work on a Win11 machine, and so admin privileges are no longer needed with Windows 11. This article also claims this to be true: https://www.howtogeek.com/16226/complete-guide-to-symbolic-links-symlinks-on-windows-or-linux/

So I'm guessing those tests do something like if ANY_WINDOWS && NO_ADMIN_PRIVS and just give up when those are true at the same time.

[alanmun]

@ac360 Hello, any chance this could get looked at, anything else I need to do? My team and I depend on this functionality for our project. @ashwinsaval