Merge | SqlCommand Execute Xml (#3637)

* Add partial for XML methods

* Merge ExecuteXmlReader

* Merge CompleteXmlReader

* Merge ExecuteXmlReaderAsync

* Merge InternalExecuteXmlReaderWithRetryAsync

* Merge InternalExecuteXmlReaderAsync

* Merge BeginExecuteXmlReader

* Merge BeginExecuteXmlReaderAsync (which seems to be completely pointless...)

* Merge BeginExecuteXmlReaderInternal

* Merge BeginExecuteXmlReaderInternalReadStage

* Merge EndExecuteXmlReader

* Merge EndExecuteXmlReaderInternal and CleanupAfterExecuteXmlReaderAsync

* Merge EndExecuteXmlReaderAsync

* Merge execute xml context internal class (and fix broken xml comments link)

# Conflicts:
#	src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlCommand.netcore.cs
#	src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/SqlCommand.netfx.cs

* Add comment for partial

Author: benrr101

src/Microsoft.Data.SqlClient/netcore/src/Microsoft.Data.SqlClient.csproj

@@ -588,6 +588,9 @@
     <Compile Include="$(CommonSourceRoot)Microsoft\Data\SqlClient\SqlCommand.Scalar.cs">
       <Link>Microsoft\Data\SqlClient\SqlCommand.Scalar.cs</Link>
     </Compile>
+    <Compile Include="$(CommonSourceRoot)Microsoft\Data\SqlClient\SqlCommand.Xml.cs">
+      <Link>Microsoft\Data\SqlClient\SqlCommand.Xml.cs</Link>
+    </Compile>
     <Compile Include="$(CommonSourceRoot)Microsoft\Data\SqlClient\SqlCommandSet.cs">
       <Link>Microsoft\Data\SqlClient\SqlCommandSet.cs</Link>
     </Compile>