Creates a campaign.

A campaign must belong to a workspace. Although workspace is optional in the request schema, set workspace.id to a workspace id from the account's session context (GET /startup) — omitting it fails with WORKSPACE_VALIDATION_ERROR ('Workspace not valid'). The workspace must also be consistent with the entities the campaign references (traffic source, flow, offers, landers): a campaign in one private workspace cannot reference entities in a different private workspace.