[Fleet] Allow user to change output plain value to secret in the UI

[jillguyonnet]

Relates to #157458

With the existing UI for managing output secrets in Fleet (#169429), once an output uses a plain value for a field that could use a secret instead, there is no way to convert that to a secret via the UI. This task should provide a way to that.

NB: this is considered a nice-to-have, not a blocker for #157458.

Suggestions:

• When a field that could be secret uses a plain value, show a notification to offer the user to change it to a secret with a CTA (button) to do that.
• Notify the user that they won't be able to see the value once it is stored as a secret (either with the CTA above or as a confirmation modal before going ahead with the change).

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)