Add a basic implementation of the applyCampaign mutation

Created by: mrnugget

This is another PR to be merged into https://github.com/sourcegraph/sourcegraph/pull/11675 and adds a basic implementation of the applyCampaign mutation.

What the mutation already does:

• Find/create a campaign based on the given namespace and the name of the given campaignSpec.
• Transfer fields from the given campaignSpec to the campaign — I'm not sure whether we want to keep that. I can see some pros in copying the data, but I don't think it's necessary, since to compute the "desired state" we'll need to have the campaign, campaignSpec and changesetSpecs at hand anyway.
• handle the ensureCampaign parameter

What I'm not sure about:

• Copying fields, mentioned above.
• why we need a namespace: ID! in applyCampaign, since the campaignSpec has a namespace already.