[META 555] Add automated span type/subtype checking against shared spec

[SylvainJuge]

Add checking the shared JSON spec for span types & sub-types.

Relates to https://github.com/elastic/apm/issues/502

Implementation checklist

• [ ] add/modify existing tests to ensure that the spec is being enforced for tests (unit or integration), the goal is to make it transparent to any contributor.
• [ ] add to/modify existing spec to allow agent tests to pass if there is any failure
• [ ] merge the PR that will implement this
• [ ] add any inconsistency to https://github.com/elastic/apm/issues/502 description