feat: New env argument to rel_from_sql()

[krlmlr]

Closes #645.

[krlmlr]

@hannes: I'd like the code below to work. The issue is that Bind() is called twice, once during rel_from_sql() (which is fine) and once when accessing the data (how to fix?).

library(duckdb)
#> Loading required package: DBI
con <- dbConnect(duckdb(environment_scan = TRUE))

create_rel <- function() {
  df <- data.frame(a = 1)
  duckdb:::rel_from_sql(con, "FROM df")
}

# This works
rel <- create_rel()
rel
#> DuckDB Relation: 
#> ---------------------
#> --- Relation Tree ---
#> ---------------------
#> Subquery
#> 
#> ---------------------
#> -- Result Columns  --
#> ---------------------
#> - a (DOUBLE)

# This fails
duckdb:::rel_to_altrep(rel)
#> Error: Error evaluating duckdb query: Catalog Error: Table with name df does not exist!
#> Did you mean "pg_depend"?
#> 
#> LINE 1: SELECT * FROM df
#>              ^

# Next level: this still works and doesn't crash the session
invisible(gc())
invisible(gc())
invisible(gc())
duckdb:::rel_to_altrep(rel)
#> Error: Error evaluating duckdb query: Catalog Error: Table with name df does not exist!
#> Did you mean "pg_depend"?
#> 
#> LINE 1: SELECT * FROM df
#>              ^

^{Created on 2025-03-13 with reprex v2.1.1}

[krlmlr]

Need this PR for the example to work.

[krlmlr]

Proposed solution: on first bind, cache the mapping name -> SEXP somewhere.

Need to check the behavior with dbSendQuery() .