Enhance NATS provider to support multiple authentication methods

[latchmihay]

Enhance NATS provider to support multiple authentication methods including JWT, NKey, and Username/Password.

This would allow the notification-controller to support the recommended NATS 2.0+ authentication paradigms.

[cappyzawa]

Thanks for this contribution!

I noticed that this PR targets release/v1.7.x branch. New features should typically be submitted against the main branch. The release/* branches are used for backports and patch releases only.

Could you please retarget this PR to main?

[cappyzawa]

Thanks for changing the target branch!

However, the commit history still contains commits from the release/v1.7.x branch. Could you please rebase your branch onto main so that only your actual change is included?

git fetch upstream main
git rebase upstream/main
git push -f