Feat: add LDAP custom mapper resource

[NataliaKhodiakova]

Added resource to configure self-deployed custom LDAP mapper (#862)

fixes: #862

[punycode]

Today, while working on terraforming a Keycloak installation for a customer, I came across the exactly same need and already wanted to fire up my Go IDE. Then I found this PR and can totally agree with the need for such a terraform resource.

I only have one additional thing to add: such a resource should support a map backed config attribute in the same way as keycloak_generic_protocol_mapper (code) does. This would be needed for some custom LDAP mappers, that need additional configuration.

[NataliaKhodiakova]

Hi @punycode thanks for the feedback! I added the optional config attribute to the resource

[mrparkers]

Hi @NataliaKhodiakova @punycode, sorry for the delay on this. I was able to pull this down and experiment with this locally. Thanks for adding the example and tests. Everything here looks great