Added Schema Definitions for Ad Cloud DSP Campaign Placement related MetaData

**What are the schemas that are affected by the issue**

We add 5 new classes for the AdCloud DSP Metadata , related to Campaign Placement:

- Placement
- Campaign
- Package
- Advertiser
- Account

**What are examples of products that are impacted by the issue**

Adobe Advertising Cloud

Start by reviewing the repository's existing schema definitions and the XDM conventions for Adobe Advertising Cloud metadata. The issue names five classes—Placement, Campaign, Package, Advertiser, and Account—but provides no files, properties, examples, or tests; done means all five related schema definitions are added consistently.

Written by the indexing model from the issue text.