[v16] Add static host users

[atburke]

Backport #45292, #45573, #46093, and #46365 to branch/v16

Changelog: Added host users that are created out-of-band, independently of an SSH session

[public-teleport-github-review-bot]

@atburke - this PR will require admin approval to merge due to its size. Consider breaking it up into a series smaller changes.

[rosstimothy]

Let's not forget to include #46246.

Could you also update the changelog to provide a bit more detail about what static host users are? Maybe something along the lines of what is noted in the upcoming releases?

[rosstimothy]

Test Plan

• Creating user from resource
  • login doesn't exist on host -> create the user
  • login exists on host, not managed by Teleport -> do nothing
  • login exists on host, insecure-drop -> do nothing
  • login exists on host, keep -> do nothing
• More than one matcher in an SHU matches a node -> log a warning and do nothing
• Deleting SHU resource doesn't remove the user/home directory/sudo entitlements
• Automatic Host User Creation
  • keep mode doesn't update/overwrite a static user
  • drop mode doesn't update/overwrite/delete a static user

[r0mant]

Bot.