Add support for Hashicorp LDAP secrets engine

[soklakov]

According to https://docs.spring.io/spring-cloud-vault/reference/secret-backends.html and https://docs.spring.io/spring-vault/reference/vault/vault-secret-engines.html currently there is no built-in support for Hashicorp LDAP secrets engine (https://developer.hashicorp.com/vault/docs/secrets/ldap).

I'd like you to add it because Hashicorp LDAP secrets engine supports a password rotation and it would be nice to have such integration out of the box.

[wilkinsona]

Spring Cloud Vault is not part of Spring Boot. Please make your suggestion here instead.

[soklakov]

Thanks! Added this one request there spring-cloud/spring-cloud-vault#745