[FEATURE]: Allow adding users from different providers (LDAP) to local groups

[fpfuetsch]

Describe the solution you'd like
Currently the provider_type of the group is used when adding members to it. This does not allow to add users from external providers like LDAP to a local group.

Code reference

Describe alternatives you've considered
I would like to be able to specify the provider_type of a group member independently of the group's provider_type.

Additional context
Actually it's possible to add users from Active Directory to local groups by specifying the AD domain in the users name, which makes this thing less consistent.

A quick solution could be that the provider_type is not passed as a filter when adding members here.

[anupamaloke]

@fpfuetsch, thank you for submitting this feature request. I see a PR coming :-)