Add LDAP based users-groups system
This change adds support to integrate an LDAP server for user
authentication. This works via a custom STS API for LDAP. Each user
accessing the MinIO who can be authenticated via LDAP receives
temporary credentials to access the MinIO server.
LDAP is enabled only over TLS.
User groups are also supported via LDAP. The administrator may
configure an LDAP search query to find the group attribute of a user -
this may correspond to any attribute in the LDAP tree (that the user
has access to view). One or more groups may be returned by such a
query.
A group is mapped to an IAM policy in the usual way, and the server
enforces a policy corresponding to all the groups and the user's own
mapped policy.
When LDAP is configured, the internal MinIO users system is disabled.
