Add Argon2PasswordEncoder

Add PasswordEncoder for the Argon2 hashing algorithm (Password Hashing
Competition (PHC) winner).
This implementation uses the BouncyCastle-implementation of Argon2.

Fixes gh-5354

docs/manual/src/docs/asciidoc/_includes/servlet/architecture/password-encoder.adoc

@@ -265,6 +265,23 @@ String result = encoder.encode("myPassword");
 assertTrue(encoder.matches("myPassword", result));
 ----
 
+[[pe-a2pe]]
+== Argon2PasswordEncoder
+
+The `Argon2PasswordEncoder` implementation uses the https://en.wikipedia.org/wiki/Argon2[Argon2] algorithm to hash the passwords.
+Argon2 is the winner of the https://en.wikipedia.org/wiki/Password_Hashing_Competition[Password Hashing Competition].
+In order to defeat password cracking on custom hardware, Argon2 is a deliberately slow algorithm that requires large amounts of memory.
+Like other adaptive one-way functions, it should be tuned to take about 1 second to verify a password on your system.
+The current implementation if the `Argon2PasswordEncoder` requires BouncyCastle.
+
+[source,java]
+----
+// Create an encoder with all the defaults
+Argon2PasswordEncoder encoder = new Argon2PasswordEncoder();
+String result = encoder.encode("myPassword");
+assertTrue(encoder.matches("myPassword", result));
+----
+
 [[pe-pbkdf2pe]]
 == Pbkdf2PasswordEncoder