Backend: ApplicationSettings - make the new slack_app_signing_secret required if slack_app_enabled

About

A new slack_app_signing_secret property is being added to ApplicationSettings in !86623 (merged) to enable us to verify Slack requests #358676 (closed).

After the property has been added to SaaS GitLab instance #361596 (closed) we should make it a required property if the slack_app_enabled setting is true.

As SaaS is the only GitLab instance that could integrate with the GitLab Slack app, it is the only instance that needs to be updated before we can make the property required in this way.

We should do this before finishing &1211 (closed) as doing this afterwards will be much more complicated.

Proposal

• Model validation in ApplicationSetting similar to this.
• Set up a dependent argument relationship in the API params similar to this.

Note that the other slack_app_ settings in ApplicationSetting do not currently have this treatment.

We should also update the following settings to also be required in the same way:

• slack_app_id
• slack_app_secret
• slack_app_verification_token