#2060 Add fieldgroups 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 should be rebased after that one is merged.

This PR adds the fieldgroups that build upon the datatypes added in #2061. These new fieldgroups capture Paid Media domain heirarchy knowledge across ad networks (meta, linkedIn, google ads, etc) such as:

• Paid Media Accounts
• Paid Media Campaigns
• Paid Media Ad Groups/Ad Squads/Ad Sets
• Paid Media Ads and Experiences
• Paid Media Assets (videos, images, etc)

It also captures performance metrics concepts, which can be reused 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.

This PR also adds a validation script ("npm run validate") to ensure example files actually adhere to their associated schemas, and that schema "$id" references to other remote schemas actually refer to either:

1. an unpublished schema $id that is actually present in the repository (useful for introducing new unpublished schemas in PRs that will still be evaluated for schema adherence)
2. a published external schema that can be fetched via http/s and included for schema adherence evaluation