Initial implementation of the serializer for Table API Support.

[prashastia]

Table API support requires conversion of Row Data to Protobuf for writing to BigQuery via the Storage Write API. This PR deals with the initial implementation for the same.

Note:

• BIGNUMERIC , DATETIME and TIME Data is not supported yet. (Will be done in future, since Read needs to be fixed.)
• The serializer will not support INTERVAL_YEAR_MONTH, INTERVAL_DAY_TIME, MAP, MULTISET, NULL, SYMBOL, RAW, DISTINCT_TYPE, STRUCTURED_TYPE, TIMESTAMP_WITH_TIME_ZONE in the MVP release of Table API Support.

/gcbrun

[jayehwhyehentee]

@clmccart requesting your review on this PR. Thanks!

[prashastia]

@yerramalli requesting your review on this PR. Thanks!