Add pmin pmax function support for sql server

[eric-fey-hus]

SQL Server (Transact SQL) support for pmin and pmax functions can easily be added.

Lines 295f in tidyverse dbplyr/R/backend-mssql.R state pmin, pmax not supported. But SQL Server (Transact SQL supports this:
SQL Server (Transact SQL) supports LEAST and GREATEST as already implemented in backend-.R,
see https://learn.microsoft.com/en-us/sql/t-sql/functions/logical-functions-least-transact-sql?view=sql-server-ver16

Therefore I propose to remove

      pmin          = sql_not_supported("pmin"),
      pmax          = sql_not_supported("pmax"),

which allows dbplyr/R/backend-.R lines 217f covers to work as expected

      pmin = sql_aggregate_n("LEAST", "pmin"),
      pmax = sql_aggregate_n("GREATEST", "pmax"),

and that is supported in SQL Server and Transact SQL.

Tested this locally on my fork using DBI and odbc: "ODBC Driver 18 for SQL Server", and it worked.

Can I create a pull request?

[eric-fey-hus]

And here would be a test:

# Using testthat:
test_that("pmin pmax functions translated correctly", {
  local_con(simulate_mssql())

  expect_equal(test_translate_sql(pmin(date1, date2, na.rm = TRUE)),
               sql("LEAST(`date1`, `date2`)"))
})