Flush rules to be cognizant of in-memory database size

[Tang8330]

Addresses #68

This is necessary because Snowflake and BigQuery both have their limits on how big (in kb) a request can be. Snowflake's API docs specify 1 MB src...but the actual limit is significantly higher.

Through this PR, I've tested 25 mb payloads without any issue. However, this does happen...especially for tables with TOAST columns. The current workaround is to just reduce the number of items from the bufferRow but that's not robust.

This PR will allow users to specify an additional merge rule, and our merge rules will now look like:

1. Flush interval time (default 10s)
2. Pool rows (default to 15k) - number of rows.
3. Pool size (default to 25 mb) - size of the in-memory db.

Whichever one is invoked first will trigger a flush cycle.

TODOs:

• Update documentation -> https://docs.artie.so/running-transfer/options
• Provide benchmarking
• Tests
• Impl

[Tang8330]

Will confirm this alleviates the REQUEST ENTITY error from Snowflake.

[Tang8330]

FYI @TamasNo1 - this is now officially supported.

[TamasNo1]

Amazing, well done @Tang8330 🎉