Support JSON data type

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](https://cloud.google.com/bigquery/docs/reference/standard-sql/data-types#orderable_data_types), 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