feat(c/driver/postgresql): Add enum type support

[paleolimbot]

Closes #1483.

This was pretty straightforward because the COPY representation is the same as a string.

I also moved all the COPY examples to the common header (since only a few were missing and it seemed odd that some of them were in different spots).

From R:

library(adbcdrivermanager)

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

db |> 
  execute_adbc("CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy')")

db |> 
  adbc_database_release()

# Have to recreate the database after because that's where we cache the types
con <- db <- adbc_database_init(
    adbcpostgresql::adbcpostgresql(),
    uri = "postgresql://localhost:5432/postgres?user=postgres&password=password"
  ) |> 
  adbc_connection_init()

con |> 
  execute_adbc("CREATE TABLE x (a mood)") |> 
  execute_adbc("INSERT INTO x VALUES ('sad')") |> 
  execute_adbc("INSERT INTO x VALUES ('ok')")

# Returns as string because that's how the driver sees it
con |> 
  read_adbc("SELECT * from x") |> 
  as.data.frame()
#>     a
#> 1 sad
#> 2  ok

^{Created on 2024-01-23 with reprex v2.0.2}

[lidavidm]

Thanks!