Adding a stale bot for ticket hygiene.

[a4z]

I do not believe having issues and PRs open forever is a good practice
But my manual intervention was not always appreciated since it could be seen as a personal action against an activity.

Therefore, I added the stale application to cross-langauage-cpp.
With it, we have a democratic robot that threads all issues and PRs equally by definition, and personal feelings against a person doing the ticket hygiene will be avoided.

After 60 days of inactivity, a message is triggered
After 7 additional days, the issue or PR gets closed,

Closed items can be reopened if ever needed.
Auto-closed issues and PRs get the label stale so we can review staled items if needed.
Exceptions: If a ticket / PR has the label idea, it will not be handled by this bot.

This PR activates the stale bot for the generator repository.

[a4z]

So, let's see what this bot will do, or not :-)