Add secrets management (Create / List / Remove) - #1224

[INFO] There are 10 errors reported by Checkstyle 6.11.2 with src/test/resources/checkstyle/checkstyle-config.xml ruleset.
[ERROR] src/main/java/com/github/dockerjava/jaxrs/CreateSecretCmdExec.java:[3,8] (imports) UnusedImports: Unused import - com.fasterxml.jackson.core.JsonProcessingException.
[ERROR] src/main/java/com/github/dockerjava/jaxrs/CreateSecretCmdExec.java:[4,8] (imports) UnusedImports: Unused import - com.fasterxml.jackson.databind.ObjectMapper.
[ERROR] src/main/java/com/github/dockerjava/jaxrs/CreateSecretCmdExec.java:[7,8] (imports) UnusedImports: Unused import - com.github.dockerjava.api.model.SecretSpec.
[ERROR] src/main/java/com/github/dockerjava/jaxrs/CreateSecretCmdExec.java:[12,8] (imports) UnusedImports: Unused import - javax.ws.rs.client.Entity.
[ERROR] src/main/java/com/github/dockerjava/api/model/SecretSpec.java:[15,8] (imports) UnusedImports: Unused import - java.util.List.
[ERROR] src/main/java/com/github/dockerjava/api/command/ListSecretsCmd.java:[3,8] (imports) UnusedImports: Unused import - com.github.dockerjava.api.model.Container.
[ERROR] src/main/java/com/github/dockerjava/api/command/ListSecretsCmd.java:[5,8] (imports) UnusedImports: Unused import - com.github.dockerjava.api.model.Service.
[ERROR] src/main/java/com/github/dockerjava/api/command/ListSecretsCmd.java:[8,8] (imports) UnusedImports: Unused import - java.util.Collection.
[ERROR] src/main/java/com/github/dockerjava/netty/NettyDockerCmdExecFactory.java:[16,8] (imports) UnusedImports: Unused import - com.github.dockerjava.api.command.ListSecretsCmd.
[ERROR] src/main/java/com/github/dockerjava/core/command/ListSecretsCmdImpl.java:[5,8] (imports) UnusedImports: Unused import - com.github.dockerjava.api.model.Service.