Add integration tests for WasmPlugin

Our current acceptance tests in maistra-test-tool only use ServiceMeshExtension to test Wasm plugin functionality. We should add some integration tests that use the new WasmPlugin API.

• Create a maistra-2.2 branch on maistra/header-append-filter that creates a valid WasmPlugin container
• Write an integration test in maistra/istio that uses header-append-filter