[Test Proxy] Support AARCH64 platform (#43428)

Author: mccoyp

eng/tools/azure-sdk-tools/devtools_testutils/proxy_startup.py

@@ -69,6 +69,12 @@
             "file_name": "test-proxy-standalone-linux-arm64.tar.gz",
             "executable": "Azure.Sdk.Tools.TestProxy",
         },
+        "AARCH64": {  # AARCH64 and ARM64 are synonyms; use the ARM binary
+            "system": "Linux",
+            "machine": "AARCH64",
+            "file_name": "test-proxy-standalone-linux-arm64.tar.gz",
+            "executable": "Azure.Sdk.Tools.TestProxy",
+        },
     },
     "Darwin": {
         "X86_64": {