Add semantic conventions

[plantfansam]

Per open-telemetry/community#1276 (comment), this PR adds semantic conventions so the library can be more spec-compliant. Users can use these modules to generate spec-compliant telemetry, e.g.:

local c = require("opentelemetry/semantic_conventions/trace/cloudevents")
tracer:start(context, name, {kind = span_kind.server, attributes = {attr.string(`c.CLOUDEVENTS_EVENT_ID`, "foo")}}

Addresses #65