feat(pkg-r): infer table_name from a data frame data_source

[cpsievert]

This PR makes the table_name parameter optional when creating a QueryChat object with a data frame. The table name is now automatically inferred from the variable name, simplifying the API for the most common use case.

Before:

qc <- QueryChat$new(mtcars, "mtcars")

After:

qc <- QueryChat$new(mtcars)  # table_name inferred as "mtcars"

The table_name parameter remains required for database connections.