#2060 Add summary metrics and lookup schemas and examples supporting Paid Media

[ejsuncy]

Issue #2060 will involve multiple PRs to keep the review effort manageable.

This stacked PR should not be merged before PR #2061 and PR #2063, and it should be rebased after each of those is merged.

This PR adds the global schemas that incorporate the fieldgroups added in #2063. These new schemas support Paid Media content metadata and content/ad performance metrics knowledge across ad networks (meta, linkedIn, google ads, etc). Schemas added include:

• Paid Media Account Metadata Lookup
• Paid Media Campaign Metadata Lookup
• Paid Media Ad Group Metadata Lookup
• Paid Media Ad Metadata Lookup
• Paid Media Experience Metadata Lookup
• Paid Media Asset Metadata Lookup
• Paid Media Summary Metrics

The Summary Metrics schema supports performance metrics at the various hierarchy levels mentioned above. For example, clicks/impressions performance metrics might apply at the Asset level, Ad level, Ad Group level, or Campaign level.