fix(toolkit): attach lifetime to generated ssh keys.

[Hweinstock]

Problem

For connecting VSCode to EC2 instance, we generate an ssh key pair on disk. This results in writing the private ssh key to VSCode global storage, allowing the key to be potentially reused by other users on the same machine.

Solution

Attach a lifetime to any key pair generated such that they wipe from disk after X seconds. Value is currently set is 30 seconds to allow connection to reliably establish. Also, change file permissions to read/write owner only and change behavior to overwrite existing keys.

This is in-line with how ec2 instance connect works: https://github.com/aws/aws-ec2-instance-connect-cli/blob/master/ec2instanceconnectcli/EC2InstanceConnectKey.py

---

License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

This pull request implements a feature or fix, so it must include a changelog entry. See CONTRIBUTING.md#changelog for instructions.

[justinmk3]

Attach a lifetime to any key pair generated such that they wipe from disk after X seconds. Value is currently set is 30 seconds to allow connection to reliably establish. Also, change file permissions to read/write owner only and change behavior to overwrite existing keys.

Great idea. Would also be a good time to add a EC2-specific section here:

aws-toolkit-vscode/docs/arch_features.md

Line 14 in 791ae55

## Remote connect

That way, we have a high-level description of the steps that happen.

[justinmk3]

Nice approach. (encapsulating this in the existing class)

[justinmk3]

If it's not much trouble, it would be helpful to remove this and use our fs.ts module instead. We want to eliminate 'fs-extra'