Support join in dbplyr query

[eitsupi]

Seems we should impliment auto_copy.tbl_polarssql_connection.
https://github.com/tidyverse/dbplyr/blob/388a6eef0e634efa693d809061ceac91d3b69a77/R/verb-copy-to.R#L97-L101

Something like:

auto_copy.tbl_polarssql_connection <- function(x, y, copy = FALSE, ...) {
  copy_to(x$src, as_polars_lf(y), unique_table_name(), ...)
}

Since the type of y is unknown, we need a generic function as_polars_lf here and need to convert it to polars LazyFrame.

[eitsupi]

dbplyr generates SQL like this by default:

SELECT "band_members".*, "plays"
FROM "band_members"
LEFT JOIN "band_instruments"
  ON ("band_members"."name" = "band_instruments"."name")

But Polars doesn't seem to support () after ON.

[eitsupi]

But Polars doesn't seem to support () after ON.

Maybe came from here.
https://github.com/pola-rs/polars/blob/447e750d338b904a190f824049af3087d837afbc/crates/polars-sql/src/sql_expr.rs#L690-L728