Allow Batch Group Membership Updates via API

Add support for adding or removing a user to/from multiple groups in a single request.
This would simplify automation and reduce the number of API calls required when provisioning or updating user access.

Edit:
Re-read that and realized I may have been unclear.
While there is a method to 'Bulk assign users to groups' I suggest adding an option to edit groups for user.

example:
I want **username** to be assigned groups: **group0**, **group2**, **group3**, **group4**
But **username** already has groups: **group1**, **group2**, **group5**
I want **group1** and **group5** to be removed, and **group0**, **group3**, **group4** to be added by a single API call e.g.:
`POST /api/v1/user//groups`
```
{
"groups": [
"group0",
"group2",
"group3",
"group4"
]
}
```