Add proper support for LDAP

Previously, we implemented LDAP auth mechanism in core tarantool. This MR aims to add support for LDAP to picodata connect and a few other api methods, namely pico.create_user & pico.change_password. It also fixes a few bugs discovered in the progress.

Related: picodata/tarantool!82.

src/schema.rs

@@ -196,9 +196,11 @@ pub struct AuthDef {
 }
 
 ::tarantool::define_str_enum! {
+    #[derive(clap::ArgEnum)]
     pub enum AuthMethod {
         ChapSha1 = "chap-sha1",
         MD5 = "md5",
+        Ldap = "ldap",
     }
 }