Include "EGH update timestamp" in metadata tables

[fccoelho]

Our metadata tables currently contain descriptions about the dataset in a table and its meaning.

However, for many datasets stored in EGH, we have periodically updated data.

This issue proposes to add an extra entry in the metadata tables called epigraphhub_timestamp that will record the most recent date that dataset was updated.

Open to other suggestions about how to accomplish this

[eduardocorrearaujo]

I think about to possibilities:

1. Add a new column in the metadata table that would maintain this information.

2. Create a new table with three columns: table_name, schema, and epigraphhub_timestamp. So we could see the updated data of all the datasets together.