Add a JSON schema

[martincostello]

To aid local authoring of a security insights file, it would be beneficial to have a JSON schema document that can be referenced via a comment in the top of the YAML file so that authors can "shift-left" the discovery of any incorrect entries.

• [Repo] Fix SECURITY-INSIGHTS open-telemetry/opentelemetry-dotnet#7144
• [Repo] Fix SECURITY-INSIGHTS open-telemetry/opentelemetry-dotnet#7145
• [Repo] Update security-insights.yml open-telemetry/opentelemetry-dotnet#7149