Feature: Variant annotations


Hi everyone,
  This is a pull request for adding an Avro IDL file for variant annotations (annotations.avdl). 
  Annotations include records for variant effect, impact, coding and non-coding information, 
clinical relevance, etc.
  It is heavily based on VEP (ENSEMBL's Variant Effect Predictor) and SnpEff (an annotations 
tool I wrote).

atm, each GAAnnotationVariant has a variantId field, so this VariantList field shouldn't be necessary.

the best fit here is probably a /variants/<variantid>/annotations method which returns an array<GAAnnotationVariant> result ([this pr](https://github.com/ga4gh/schemas/pull/120/files) is probably the closest pattern to copy from)