Add support for app role assignments to groups

[michaljirman]

This PR adds basic support for app role assignments to a group:

• list group's assigned app roles
• remove assigned app role from a group
• assign app role to a group

Resources:

• https://docs.microsoft.com/en-us/graph/api/group-post-approleassignments?view=graph-rest-1.0&tabs=http

[manicminer]

Thanks for this @michaljirman, this LGTM!

[michaljirman]

@manicminer
Np, thanks for merging. Btw. have you received any of my emails?

[manicminer]

@michaljirman yes i did, sorry for the delay but I replied just now :)