ci: add AWS CodeBuild buildspec for quic-attack

[WesleyRosenblum]

Description of changes:

This change adds an AWS CodeBuild buildspec to allow for quic-attack to be executed in CodeBuild.

Call-outs:

I made a few changes to the run script based on testing this out:

• Added -C panic=abort option to the build, since I noticed that panics in the client were not exiting the process, leaving the overall testing continuing to run until the timeout
• Added +stable to the ultraman installation so CodeBuild would not have to download another rust toolchain
• Used pkill after the ultraman command completes, since I noticed that the s2n-quic-qns processes would frequently still be running afterwards
• Capture the exit code from the timeout command and only consider it a success if the time limit was reached.

Testing:

Tested manually in CodeBuild

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.