Update version: 1.51.0 -> 1.52.0

Features
* #1832 Helper for loading Client Secrets directly from file.
* #1838 and #1874 Support for credential impersonation.
* #1851 Make it easier to configure a credential's HTTP client, in particular it's now easier to set a proxy to be used for token fetching.

Fixes
* #1874 ServiceAccountCredential should copy scopes from initializer.

Dependencies
* #1842 System.Net.Http to 4.3.4
* #1873 Add target for .NET 4.6.1

Src/Support/CommonProjectProperties.xml

@@ -2,7 +2,7 @@
 
   <!-- common nupkg information -->
   <PropertyGroup>
-    <Version>1.51.0</Version>
+    <Version>1.52.0</Version>
     <Authors>Google LLC</Authors>
     <Copyright>Copyright 2021 Google LLC</Copyright>
     <PackageTags>Google</PackageTags>

features.json

@@ -1,8 +1,8 @@
 {	
   "language": "csharp",	
   "description": "C# libraries for Google APIs.",	
-  "releaseVersion": "1.51.0", "comment1": "Version of generated package.",	
-  "currentSupportVersion": "1.51.0", "comment2": "Version of support library upon which to depend.",	
+  "releaseVersion": "1.52.0", "comment1": "Version of generated package.",	
+  "currentSupportVersion": "1.52.0", "comment2": "Version of support library upon which to depend.",	
   "pclSupportVersion": "1.25.0", "comment3": "Version of PCL support library.",	
   "net40SupportVersion": "1.10.0", "comment4": "Version of net40 support library.",