JDBC driver cannot support TIMESTAMP data type

Avatica allows column representation to be customized, so a timestamp can be stored as a variety of types. Joda ReadableInstant is none of these types: https://github.com/apache/calcite-avatica/blob/acb675de97b9b0743c09368820a770e2ceda05f8/core/src/main/java/org/apache/calcite/avatica/util/AbstractCursor.java#L162

By default, it seems to be configured to store TIMESTAMP columns as long values. If you run the SQL shell and select a TIMESTAMP column, you get:

ava.lang.ClassCastException: org.joda.time.Instant cannot be cast to java.lang.Number
        at org.apache.beam.repackaged.beam_sdks_java_extensions_sql.org.apache.calcite.avatica.util.AbstractCursor$NumberAccessor.getNumber(AbstractCursor.java:726)
        at org.apache.beam.repackaged.beam_sdks_java_extensions_sql.org.apache.calcite.avatica.util.AbstractCursor$TimestampFromNumberAccessor.getString(AbstractCursor.java:1026)
        at org.apache.beam.repackaged.beam_sdks_java_extensions_sql.org.apache.calcite.avatica.AvaticaResultSet.getString(AvaticaResultSet.java:225)
        at sqlline.Rows$Row.<init>(Rows.java:183)

So, essentially, Beam SQL Shell does not support timestamps.

We may be able to:

• override how the accessor for our existing storage is created
• configure what the column representation is (this doesn't really help, since none of the choices are ours)
• convert timestamps to longs in BeamEnumerableConverter; not sure how many conversions will be required here