Add security context holder Kotlin samples to docs

Issue gh-8172

Author: eleftherias

docs/manual/src/docs/asciidoc/_includes/servlet/authentication/architecture/security-context-holder.adoc

@@ -16,7 +16,8 @@ The simplest way to indicate a user is authenticated is to set the `SecurityCont
 
 .Setting `SecurityContextHolder`
 ====
-[source,java]
+.Java
+[source,java,role="primary"]
 ----
 SecurityContext context = SecurityContextHolder.createEmptyContext(); // <1>
 Authentication authentication =
@@ -25,6 +26,16 @@ context.setAuthentication(authentication);
 
 SecurityContextHolder.setContext(context); // <3>
 ----
+
+.Kotlin
+[source,kotlin,role="secondary"]
+----
+val context: SecurityContext = SecurityContextHolder.createEmptyContext() // <1>
+val authentication: Authentication = TestingAuthenticationToken("username", "password", "ROLE_USER") // <2>
+context.authentication = authentication
+
+SecurityContextHolder.setContext(context) // <3>
+----
 ====
 
 <1> We start by creating an empty `SecurityContext`.
@@ -40,14 +51,25 @@ If you wish to obtain information about the authenticated principal, you can do
 
 .Access Currently Authenticated User
 ====
-[source,java]
+.Java
+[source,java,role="primary"]
 ----
 SecurityContext context = SecurityContextHolder.getContext();
 Authentication authentication = context.getAuthentication();
 String username = authentication.getName();
 Object principal = authentication.getPrincipal();
 Collection<? extends GrantedAuthority> authorities = authentication.getAuthorities();
 ----
+
+.Kotlin
+[source,kotlin,role="secondary"]
+----
+val context = SecurityContextHolder.getContext()
+val authentication = context.authentication
+val username = authentication.name
+val principal = authentication.principal
+val authorities = authentication.authorities
+----
 ====
 
 // FIXME: add links to HttpServletRequest.getRemoteUser() and @CurrentSecurityContext @AuthenticationPrincipal