Add possibility to delete default mappers for a ldap user federation resource

[gerbermichi]

This pull requests resolve the issue #729.

The property default_mappers was added to the keycloak_ldap_user_federation indicating whether the default mappers should be created or not. The default is true.

Closes #729

[mrparkers]

Hey @gerbermichi, thanks for opening this PR. I had a few changes I wanted to make, but I was unable to push them to your branch, so I opened #744 for them.

Here's a summary of the changes:

• I removed the usage of the ComponentType structs nested within each mapper struct. I understand that you did this so you could genericize the deletion of these mappers, but the component struct was already used for a similar purpose, so I just re-implemented the deletion logic within the keycloak package and invoked it from the provider package.
• I renamed the new attribute to delete_default_mappers, because it feels better for the user to explicitly opt-in for this behavior.
• I added a test for this change

I'm going to close this in favor of #744. It'll be included within the next release, which I want to get out by the end of the week.