fix: google.cloud.gcp_bigquery_table clustering fields

[gomesfernandes]

SUMMARY

BigQuery table with clustering creation returns error

Fixes #620

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

gcp_bigquery_table

ADDITIONAL INFORMATION

The BigQuery API expects the clustering parameters to be sent with this format:

{
  "clustering": {
    "fields": ["field1", "field2"]
  }
}

They are currently being sent in this format (no fields):

{
  "clustering": ["field1", "field2"]
}