Add additional timestamps to BigQuery table

[hernandezc1]

In order to create metrics of the processing time of alerts in our pipeline for the ELAsTiCC2 challenge, additional parameters need to be added into the BigQuery table.

The following timestamps are added in this PR:

• elasticcPublishTimestamp
• brokerIngestTimestamp
• classifierTimestamp (previously just called timestamp)

The following files are modified:

• main.py
• bq_elasticc_SuperNNova_schema.json

These updates have been tested. A classifier instance with testid = timestamp was deployed (with trigger_topic = elasticc-loop), and the classifications were written to the following BigQuery table: elasticc_alerts_timestamp

[hernandezc1]

@troyraen I was able to include the alertId in the BigQuery table schema. Here's the BigQuery table containing the results of my testing

[troyraen]

Fixes #3