bigquery: SchemaFromJSON doesn't support policyTags

[Nabil372]

Hi there!

SchemaFromJSON ignores policyTags in JSONs which are generated by protoc-gen-bq-schema.
This function no longer does what it's described to do:
"SchemaFromJSON takes a JSON BigQuery table schema definition (as generated by https://github.com/GoogleCloudPlatform/protoc-gen-bq-schema) and returns a fully-populated Schema."

Example JSON taken from protoc-gen-bq-schema:

[
 {
  "name": "a",
  "type": "INTEGER",
  "mode": "REQUIRED",
  "policyTags": {
   "names": [
    "private"
   ]
  }
 },
 {
  "name": "b",
  "type": "STRING",
  "mode": "NULLABLE",
  "policyTags": {
   "names": [
    "public"
   ]
  }
 },
 {
  "name": "nested",
  "type": "RECORD",
  "mode": "REQUIRED",
  "fields": [
   {
    "name": "a",
    "type": "INTEGER",
    "mode": "REQUIRED",
    "policyTags": {
     "names": [
      "private"
     ]
    }
   },
   {
    "name": "b",
    "type": "STRING",
    "mode": "NULLABLE"
   }
  ]
 }
]

The function is still able to produce a bigquery.Schema but it ignores the policyTags.

[shollyman]

Will take a look at this as a FR.