Allow for dynamic columns, instead of one stringified column for the message

[RLRabinowitz]

Currently there are only 3 columns: id, timestamp and message. The message is always a JSON containing the field message, and sometimes contains other fields. Maybe we should allow for expanding each of those to its own column? That would allow customers to define GSIs on those columns and query by those columns.

This would require:

• Stringifying the values of those fields
• Removing lines with values that are empty, or empty strings, since DynamoDB doesn't allow that
• Changing the event size logic a bit (we need to check of this is true. This way would probably make each item smaller, because of how DynamoDB calculates item size)