Add "Upgrade Testing" after v0.3

[mathetake]

• Create a directory tests/e2e-upgrade and the file tests/e2e-upgrade/e2e_upgrade_test.go
• Adds a make recipe make test-e2e-upgrade
• In the tests suite
  • Setup the k8s environment as usual except that instead of installing Envoy AI Gateway via helm locally, install registry-1.docker.io/envoyproxy/ai-gateway-helm --version 0.3.0. This might require changes in tests/internal/e2elib.
  • Adds a test case where continuously making requests to a gateway with some test manifest with testupstream server etc. (You can reference tests/e2e/testdata directory.
  • While making requests, upgrade Envoy AI Gateway using the local helm under menifests/charts directory, and restart the controller.
  • During the upgrade, ensures all requests are successful for 2 minutes.