First attempt at genericizing data source (improved!)

[chendaniely]

(Moved TODO items into #21 and #22).

Squash merged into #20 via c6a24a7

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Original PR comment:

Updates, changes, fixing conflicts from #5

TODO:

• create more examples with different database engines (e.g., snowflake, databricks, etc)
• for snowflake use the user:pass string, but also show example for using snowflake.connector.connect
• Check readme for new api changes that take dataframe or sqlalchemy connection
• Check readme for new api changes of returning a querychat object not dict

From the #5 TODO:

• Use narwhals instead of Pandas directly
• Have options to limit the number of rows retrieved, or perhaps retrieve in chunks or a cursor, so big databases won't blow up your Shiny app

[chendaniely]

closing. manually squash merged into #20 via c6a24a7 to preserve Joe as the author of the changes.