FR: users API endpoint

### What are you trying to do?

As an admin I would like to be able to programmatically get lists of, and remove, users from my tailnet.

### How should we solve this?

Add a users API endpoint that accepts both GET and DELETE calls, much like the current device API. Deletion of a user should effectively also delete all devices said user has authenticated.

### What is the impact of not solving this?

I've noticed users no longer with my company showing up as inactive under the Users tab in the wed admin interface. This would be great to help keep the user base trimmed down and accurate against our authenticating IdP.

### Anything else?

Proposed data object structure (json):

`{
"id": 999999999,
"email": "someuser@somedomain.com",
"identitySource": "idp_name",
"joined": "2022-10-17T10:11:32Z",
"lastSeen": "2022-10-17T17:11:49Z",
"name": "Some User",
"firstName": "Some",
"lastName": "User"
}`