Support JSON data type

[tilschuenemann]

First of all: Thank you for maintaining this project, it really is a nice addition to pandas!

Is your feature request related to a problem? Please describe.
GBQ supports the JSON data type, but unfortunately pandas-gbq doesn't allow writing (and I believe reading) from a table that features a JSON data type column.

Describe the solution you'd like

1. Add support for both reading and writing dataframes that feature JSON columns.
2. Add support for manually specifying JSON columns in the table schema.

Describe alternatives you've considered
Alternatives are:

• writing data as a string and then loading it as a JSON inside a SQL query to access its key-values
  • the downside here is that the json object can't be used for early query processing, introducing the conversion overhead
• using the native google.cloud.bigquery module

[subramad]

Is there any update on this?