Create submodules for additional topics

[timtebeek]

• [x] Gateway token exchange with Spring Cloud Gateway + Keycloak #6, #12
• [x] Auditing Spring Data Entities #1
• [x] Restricting Spring Data Queries to active user (Pre/PostAuthorize or PostFilter Spring Data Repositories, active user in Query) #4
• [x] Custom PermissionEvaluator for group based access #7
• [x] Custom AclVoter for @Secured method arguments #8
• [x] JWT validation in resource servers (2 dependencies + 1 property in Spring Security 5.1+) #12
• [x] Testing with Spring Security Test (WebMvcTest, SpringBootTest) #14

[timtebeek]

Possible additional topics, not covered so far:

• [ ] spring-security-acl database & voter
• [ ] Creating and testing with custom security annotations
• [ ] Multi tenancy with separate databases
• [ ] Multi tenancy with discriminator columns
• [ ] Reporting AuthorizationFailureEvents + metrics
• [ ] AspectJ based method security
• [ ] Security headers in the browser
• [ ] RateLimit API events per user