fix(plugin-persistence-ethereum): make created_at TIMESTAMPTZ in schema

[petermetz]

1. The problem was that the database schema was defined in a way that was
   destroying timestamp information during insertion of records.
2. Updating the schema to hold the timestamp information made the test pass.

More information about why it's recommended to store datetime data with
the TIMESTAMPTZ column type is explained by the author of the node-postgres
library which is an important part of the problem (it assumes local time
for columns that do not store the timestamp time zone).

https://node-postgres.com/features/types#date--timestamp--timestamptz

Fixes #3373

Signed-off-by: Peter Somogyvari [email protected]

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