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Added new configuration option to ldap auth backend - groupfilter.
GroupFilter accepts a Go template which will be used in conjunction with
GroupDN for finding the groups a user is a member of. The template will
be provided with context consisting of UserDN and Username.
Simplified group membership lookup significantly to support multiple use-cases:
* Enumerating groups via memberOf attribute on user object
* Previous default behavior of querying groups based on member/memberUid/uniqueMember attributes
* Custom queries to support nested groups in AD via LDAP_MATCHING_RULE_IN_CHAIN matchind rule
There is now a new configuration option - groupattr - which specifies
how to resolve group membership from the objects returned by the primary groupfilter query.
Additional changes:
* Clarify documentation for LDAP auth backend.
* Reworked how default values are set, added tests
* Removed Dial from LDAP config read. Network should not affect configuration.
