Spanner JSON Type Not Supported When Reading Schema

Our existing data pipeline failed when we added the new [JSON column](https://cloud.google.com/spanner/docs/working-with-json) to a Spanner table that it writes to.  The following is the stack trace.
```

Error message from worker: java.lang.IllegalArgumentException: Unknown spanner type JSON
org.apache.beam.sdk.io.gcp.spanner.SpannerSchema$Column.parseSpannerType(SpannerSchema.java:179)

org.apache.beam.sdk.io.gcp.spanner.SpannerSchema$Column.create(SpannerSchema.java:142)
org.apache.beam.sdk.io.gcp.spanner.SpannerSchema$Builder.addColumn(SpannerSchema.java:74)

org.apache.beam.sdk.io.gcp.spanner.ReadSpannerSchema.processElement(ReadSpannerSchema.java:63)
```

I included a proposed patch below that has not yet been tested.

https://github.com/apache/beam/compare/master...vilai-paypal:spanner_json_schema?expand=1

Imported from Jira [BEAM-13470](https://issues.apache.org/jira/browse/BEAM-13470). Original Jira may contain additional context.
Reported by: vilai.