Allows override target version file (#8158)

* allow an override FILE to be honored by test-proxy-tool
* remove test-proxy-docker

Author: scbedd

eng/common/testproxy/test-proxy-docker.yml

@@ -22,7 +22,17 @@ steps:
       pwsh: true
 
   - pwsh: |
-      $version = $(Get-Content "${{ parameters.templateRoot }}/eng/common/testproxy/target_version.txt" -Raw).Trim()
+      $standardVersion = "${{ parameters.templateRoot }}/eng/common/testproxy/target_version.txt"
+      $overrideVersion = "${{ parameters.templateRoot }}/eng/target_proxy_version.txt"
+
+      $version = $(Get-Content $standardVersion -Raw).Trim()
+
+      if (Test-Path $overrideVersion) {
+        $version = $(Get-Content $overrideVersion -Raw).Trim()
+      }
+
+      Write-Host "Installing test-proxy version $version"
+
       dotnet tool install azure.sdk.tools.testproxy `
         --tool-path $(Build.BinariesDirectory)/test-proxy `
         --add-source https://pkgs.dev.azure.com/azure-sdk/public/_packaging/azure-sdk-for-net/nuget/v3/index.json `