Add /examples/* to basic FastAPI #1005

For the object types listed below, provide endpoints in the basic FastAPI implementation such that:

GET /examples/<object-type> provides an example object of that type
POST /examples/<object-type> runs the provided JSON object through the pydantic validation for the object type

This will provide a simple way to bootstrap folks who want to develop code that emits valid (schema compliant) SSVC JSON objects.

The object types we need this for include:

• decision points
• decision tables
• selection lists
• Optional — because they're technically covered by their inclusion in the above, but we might as well do it anyway since it's not much extra effort
  • decision point values — an individual element of a decision point value
  • selection — an individual element of a selection list
  • minimal decision point values — an individual element of an individual selection item