Declarative specification for OpenTelemetry semantic conventions

Let's see if we can extract the logic in `processor/otel` for mapping OpenTelemetry semantic conventions to our data model into a declarative specification.

The goal of this is to automatically generate reference documentation, and to decouple the specification from the APM Server code to enable outside reuse and contributions.

The specification should either be directly and efficiently executable, or enable code generation for the mapping.