Use Marshal.SecureStringToGlobalAllocUnicode

Relates: #3806 (comment)

This commit updates the SecureStrings implementation to use the cross platform
Marshal methods that deal with unmanaged unicode strings.

Implement IDisposable on BasicAuthenticationCredentials and dispose of SecureString
instances when ConnectionConfiguration is disposed.

Author: russcam

src/Elasticsearch.Net/Configuration/ConnectionConfiguration.cs

@@ -513,6 +513,8 @@ protected virtual void DisposeManagedResources()
 			_connectionPool?.Dispose();
 			_connection?.Dispose();
 			_semaphore?.Dispose();
+			_proxyPassword?.Dispose();
+			_basicAuthCredentials?.Dispose();
 		}
 	}
 }

src/Elasticsearch.Net/Configuration/Security/BasicAuthenticationCredentials.cs

@@ -1,11 +1,12 @@
-using System.Security;
+using System;
+using System.Security;
 
 namespace Elasticsearch.Net
 {
 	/// <summary>
 	/// Credentials for Basic Authentication
 	/// </summary>
-	public class BasicAuthenticationCredentials
+	public class BasicAuthenticationCredentials : IDisposable
 	{
 		public BasicAuthenticationCredentials()
 		{
@@ -32,5 +33,7 @@ public BasicAuthenticationCredentials(string username, SecureString password)
 		/// The username with which to authenticate
 		/// </summary>
 		public string Username { get; set; }
+
+		public void Dispose() => Password?.Dispose();
 	}
 }

src/Elasticsearch.Net/Connection/SecureStrings.cs

@@ -19,13 +19,13 @@ public static string CreateString(this SecureString secureString)
 			{
 				try
 				{
-					num = Marshal.SecureStringToBSTR(secureString);
-					return Marshal.PtrToStringBSTR(num);
+					num = Marshal.SecureStringToGlobalAllocUnicode(secureString);
+					return Marshal.PtrToStringUni(num);
 				}
 				finally
 				{
 					if (num != IntPtr.Zero)
-						Marshal.ZeroFreeBSTR(num);
+						Marshal.ZeroFreeGlobalAllocUnicode(num);
 				}
 			}