WIP: Add LDAP Server API

[n1cc00]

WIP: LDAP Server API

This PR introduces initial support for the Synology LDAP Server API.

Context

The naming for the Directory / LDAP Server APIs is very confusing:

• LDAP Server endpoints are accessed via the SYNO.DirectoryServer API.
• Directory Server endpoints, however, use the SYNO.ActiveDirectory API.

What’s Included

• New endpoints:
  • list_users
  • create_new_user

Missing

Of course, there are other endpoints to be implemented.
Also, proper error handling needs to be added. I wasn't sure how to correctly match SYNO.Entry.Request, because besides Syno.ActiveDirectory (the Directory Server API), Syno.DirectoryServer (the LDAP Server API) seems to use it too. So I left that for later.

Extended parameter validation should also be added as the LDAP API seems to sometimes add invalid/incomplete entries instead of rejecting them (moving the example _assert_expireddate_validity() function to another place in the codebase along with other functions)

I hope to be able add more myself in the near future but as my time is very limited right now, I wanted to publish this draft first.

This is NOT really tested yet. Will do that in the next days.

[FBoissadier]

Just for information you can make request to "SYNO.Entry.Request" API using "batch_request" function from base_api :)
To be honest if the request you see is a "SYNO.Entry.Request" there is a strong possibility that it's use in sequential mode, so you can just implement each request one by one (one request = one function). As I know I've used it for package center API, but I forgot if it was really necessary or we can run request independant.
I didn't have the LDAP server installed on my Synology, but nice to see a new integration :)