feat(r/DataFrameSource): Use duckdb or SQLite (first available)

[gadenbuie]

Closes #161

• DataFrameSource now inherits from DBISource, since the only difference in implementation was in $initialize()
• Added engine argument and querychat.DataFrameSource.engine option to choose to use "duckdb" or "sqlite" for the in-memory database
• By default, if no explicit choice is made, we use the first available of duckdb or SQLite