Allow to display and update allow_personal_snippets via API

Problem

We want to allow users to modify Allow personal snippets settings using API.

The value of this setting is stored in database (see Add allow_personal_snippets field to group sett... (#555051 - closed)).

Proposal

• Update API to allow users to enable/disable and see this configuration.
• Update API documentation
• This functionality should be released behind the same feature flag as in Add a UI configuration option to the group sett... (#552392 - closed).

Additional Notes

The ability to toggle this setting should only be available to group owners and should only impact enterprise users in the namespace.

UI counterpart issue: Add a UI configuration option to the group sett... (#552392 - closed)

Similar implementation

Disable personal access tokens for enterprise u... (!148415 - merged)