(#23349) Create autosign setting for input validation

This commit adds adds a new 'autosign' setting for the autosign command.
There are two reasons we're doing this: first, setting validation is
best done with a new setting class, and second, defining a hook for a
setting that might interpolate an application setting can cause the
interpolation to fail. By moving validation to a setting class, we can
sidestep this issue for now.

Jira issue: PUP-932
(#23349) Defer setting hooks to post app init when needed

Defining a hook for settings can force that setting to be interpolated
before all settings are loaded, which will fail. This commit allows
setting hooks to be deferred after application initialization, so that
values can be properly interpolated before invoking the hook.

Jira issue: PUP-932
Merge branch 'pr/2127' into stable

* pr/2127:
  (#23349) Defer setting hooks to post app init when needed
  (#23349) Create autosign setting for input validation