ldap: Add user DN attributes list config param (#19758)

This change uses the updated ldap library in minio/pkg (bumped
up to v3). A new config parameter is added for LDAP configuration to
specify extra user attributes to load from the LDAP server and to store
them as additional claims for the user.

A test is added in sts_handlers.go that shows how to access the LDAP
attributes as a claim.

This is in preparation for adding SSH pubkey authentication to MinIO's SFTP
integration.

internal/config/identity/ldap/help.go

@@ -66,6 +66,12 @@ var (
 			Optional:    true,
 			Type:        "string",
 		},
+		config.HelpKV{
+			Key:         UserDNAttributes,
+			Description: `"," separated list of user DN attributes e.g. "uid,cn,mail,sshPublicKey"` + defaultHelpPostfix(UserDNAttributes),
+			Optional:    true,
+			Type:        "list",
+		},
 		config.HelpKV{
 			Key:         GroupSearchFilter,
 			Description: `search filter for groups e.g. "(&(objectclass=groupOfNames)(memberUid=%s))"` + defaultHelpPostfix(GroupSearchFilter),