LDAP Provider flatten list for user/group attributes

**Is your feature request related to a problem? Please describe.**
Currently Authentik seems to misbehave with custom mappings so that I could properly integrate it with SSSD (having multiple ssh keys)

**Describe the solution you'd like**
I would love for it to be supported.

**Describe alternatives you've considered**
N/A

**Additional context**
I have tried to do the following:
1. New prompt with field key attributes.sshPublicKey, interpret placeholder as expression:
```python
try:
keys = user.attributes.get("sshPublicKey", "")

if type(keys) is str:
return keys
else:
return "\n".join(keys)
except:
return None
```
2. New expression policy for ssh key that has the following:
```python
request.context['prompt_data'].get('attributes', {}).pop('attributes.sshPublicKey', None)
```
3. Try to lookup the user via LDAP, and see that it's empty as well as after adding it to user settings it's also empty?