Support for BIGNUMERIC type

[machow]

Hey--I've been really enjoying using ibis with bigquery! I've noticed that for bigquery BIGNUMERIC columns, it throws this error when connecting...

SyntaxError: Type cannot be parsed: BIGNUMERIC

It looks like google-cloud-bigquery has configuration for BIGNUMERIC--does this mean that supporting it in this library requires these two changes?:

• Removing this check on precision / scale:

  ibis-bigquery/ibis_bigquery/datatypes.py

  Line 112 in 2ff42c3

  @ibis_type_to_bigquery_type.register(dt.Decimal, TypeTranslationContext)

• Adding BIGNUMERIC here:

  ibis-bigquery/ibis_bigquery/client.py

  Line 45 in 2ff42c3

  'NUMERIC': dt.Decimal(38, 9),

[saschahofmann]

@loudinb did you make progress with this. Seems like you already worked on this?