[5.x] Add various events

[duncanmcclean]

This pull request implements various events, for some non-content related actions which are missing events for auditing purposes:

• ImpersonationStarted and ImpersonationEnded events
  • These events will be dispatched whenever a user starts or finishes impersonating another user. Both the $impersonator and the $impersonated users are available in the event.
• StacheCleared
  • This event will be dispatched after the Stache cache has been cleared, either via the php please stache:clear command or via the Cache Manager utility.
• StacheWarmed
  • This event will be dispatched after the Stache cache has been warmed, either via the php please stache:warm command or via the Cache Manager utility.
• StaticCacheCleared
  • This event will be dispatched after the Static cache has been cleared, via the php please static:clear command or via the Cache Manager utility.
• GlideCacheCleared
  • This event will be dispatched after the Glide cache has been cleared, either via the php please glide:clear command or via the Cache Manager utility.
• SearchIndexUpdated
  • This event will be dispatched when a search index is updated, either via the php please search:update command or via the Search utility.
  • The $index instance is available on the event.
• UserPasswordChanged
  • This event will be dispatched when the password of another user is changed via the Control Panel.
  • This event won't be dispatched when a user's password is "reset" via the CP authentication pages, or via the {{ user:reset_password_form }} tag. In this case, Laravel's PasswordReset event would be dispatched.
• LicenseSet
  • This event will be dispatched after a license key has been set via the php please license:set command.
  • This event won't be dispatched when the license key is manually added to a site's .env file.
• LicensesRefreshed
  • This event will be dispatched when a user manually triggers a "Sync" of a site's licenses via the Licenses utility.
  • This event won't be dispatched when Statamic pings the Outpost automatically, every hour.

As a note, this PR doesn't implement an event for when Laravel's application cache is cleared via the Cache Manager utility as Laravel provides its own cache:cleared event for this purpose.

Closes: #10403
Related: #10460

[jasonvarga]

Lookin' good

[johncarter-]

Useful stuff. Thanks.