feat(c/driver/postgresql): Handle NUMERIC type by converting to string

[paleolimbot]

Closes #767.

This PR implements NUMERIC Postgres columns by converting them to strings. This is not ideal but there is also no Arrow type that can be easily computed before an entire column of Postgres numerics has been resolved. A future extension could attempt decimal conversion in the driver but for now a string value is better than is what currently gets returned (a blob of the COPY data).

Example support via the R bindings:

# pak::pak("apache/arrow-adbc/r/adbcpostgresql#883")
# docker compose up postgres_test
library(adbcdrivermanager)

uri <- "postgresql://localhost:5432/postgres?user=postgres&password=password"
db <- adbc_database_init(adbcpostgresql::adbcpostgresql(), uri = uri)

db |> 
  read_adbc(
    "SELECT CAST(col AS NUMERIC) AS col FROM 
      (VALUES (-123.456), (123.456), (0.4567890), (1200000), ('nan'), ('-inf'), ('inf'), (NULL)) 
      AS drvd(col)"
  ) |> 
  as.data.frame()
#>         col
#> 1  -123.456
#> 2   123.456
#> 3 0.4567890
#> 4   1200000
#> 5       nan
#> 6      -inf
#> 7       inf
#> 8      <NA>

^{Created on 2023-07-08 with reprex v2.0.2}

[lidavidm]

nit: !special_value.empty()

[lidavidm]

is the 1 * necessary?

[paleolimbot]

Good catch! (Was a typo 😬 )