Create submodules for additional topics

[timtebeek]

• Gateway token exchange with Spring Cloud Gateway + Keycloak Pure webflux #6, Add Spring Cloud Gateway with Open ID Connect and TokenRelay example #12
• Auditing Spring Data Entities Audit spring data entities #1
• Restricting Spring Data Queries to active user (Pre/PostAuthorize or PostFilter Spring Data Repositories, active user in Query) Limit spring data queries #4
• Custom PermissionEvaluator for group based access Custom PermissionEvaluator #7
• Custom AclVoter for @Secured method arguments Custom AccessDecisionVoter #8
• JWT validation in resource servers (2 dependencies + 1 property in Spring Security 5.1+) Add Spring Cloud Gateway with Open ID Connect and TokenRelay example #12
• Testing with Spring Security Test (WebMvcTest, SpringBootTest) Adding spring security #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