Support embedded examples in package schemas

[AaronFriel]

Provider authors and bridging tools should be able to embed examples in YAML or the intermediate representation, PCL, into schemas.

• Enable examples codegen to be decoupled from bridging and provider authoring
• Enable tools to extract examples from structured fields, as opposed to text extraction from description fields

[iwahbe]

Design doc is https://docs.google.com/document/d/1CpOmG3QNtY1fGX1vuXsb5nehr40lomknbJtjWivjBhM/edit#