Watcher: Support reloadable keystore

[spinscale]

The reload-secure-store-action feature branch adds support for a reloadable keystore.

Watcher uses keystore settings for Slack, Hipchat, PagerDuty, Jira.

While at it, the Email service should also get a new secure setting for the user/password of a user.

Lastly, we should align the deprecation with of the non secure settings, how it is done with the other discovery/plugin settings.

[elasticmachine]

Pinging @elastic/es-core-infra

[ppf2]

This one seems obsolete. AFAICT, Watcher can now reload secure settings as part of its notification service (#31746)?

[dakrone]

This is obsolete, as keystore settings can now be reloaded.