[Appender] Add support for ENUM

[rayokota]

This issue is to request first-class support for ENUM in the Appender, which currently errors with Error: unsupported C API type: 23 on construction.

[staticlibs]

Hi, thanks for the report! Yes, this should be added, along with BIT and INTERVAL.

[staticlibs]

@rayokota

Just FYI, ENUMs support is added in #445. On closer look both BIT and INTERVAL appeared to require proper support in ResultSet first, and API for them is not obvious, so in Appender both are postponed.

[rayokota]

Thanks @staticlibs !