precondition check was being honored before, validating
if anonymous access is allowed on the metadata of an
object, leading to metadata disclosure of the following
headers.
```
Last-Modified
Etag
x-amz-version-id
Expires:
Cache-Control:
```
although the information presented is minimal in nature,
and of opaque nature. It still simply discloses that an
object by a specific name exists or not without even having
enough permissions.
This change uses the updated ldap library in minio/pkg (bumped
up to v3). A new config parameter is added for LDAP configuration to
specify extra user attributes to load from the LDAP server and to store
them as additional claims for the user.
A test is added in sts_handlers.go that shows how to access the LDAP
attributes as a claim.
This is in preparation for adding SSH pubkey authentication to MinIO's SFTP
integration.
Following extension allows users to specify immediate purge of
all versions as soon as the latest version of this object has
expired.
```
<LifecycleConfiguration>
<Rule>
<ID>ClassADocRule</ID>
<Filter>
<Prefix>classA/</Prefix>
</Filter>
<Status>Enabled</Status>
<Expiration>
<Days>3650</Days>
<ExpiredObjectAllVersions>true</ExpiredObjectAllVersions>
</Expiration>
</Rule>
...
```
