[1.0.0] Add a schema to the backend.

[ChadSikorra]

Now that the backend actually has persistence built-in, and we are already handling that logic, it makes sense to integrate a schema per RFC 4512. This way library users can define an explicit schema that clients must adhere to. Otherwise currently there's nothing preventing any client to write anything they want to any entry.

This also helps lay the foundation for put in some kind of ACL, or at least a decision hook, for restricting entry modifications / access based on the authenticated client.

So this does the following:

• Adds the base schema models / DTOs.
• Adds a way to set them in the server options.
• Exposes it in the sub-schema entry so it's discoverable by clients.
• Enforces schema validation on filter evaluation and writes.

Explicitly ignoring LDIF for now. I can always add it later. This was already a lot of files to properly support this.