feat: onboard the repo to the testing infra

Issue #, if available: DOTNET-7477

Description of changes:

Added GitHub workflow that runs unit tests and integ tests using the testing infrastructure on PRs targeting main, dev and feature branches.
Updated the build project to use mono to run unit tests on Linux. Previously, they only supported Windows.
Added a buildspec file that the testing infrastructure will use to run the tests. The buildspec adds mono to the base image.

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

aws / aws-lambda-dotnet