Add Timestamp support

[uberchris2]

Currently, the Firestore Timestamp data type cannot be read.

When trying to read and output as a string, the following error is produced:

Document field 'SomeTimestampField' is expected to be of type 'java.lang.String', but found a 'Timestamp'. at io.cdap.plugin.gcp.firestore.source.QueryDocumentSnapshotToRecordTransformer.ensureTypeValid(QueryDocumentSnapshotToRecordTransformer.java:158

Attempting to output as a timestamp/date/etc. produces a similar error.