fix(c/driver/postgresql): add postgres type to cols created for numeric

[lupko]

• introduce constant with key name
• introduce separate private method to add field-level method

Fixes #1515

[lidavidm]

A unit test would be appreciated but I can understand if the setup is a little difficult

[lupko]

@lidavidm thanks for checking it out. good point re the tests - will add; I see there are bunch of tests in this area already so will extend. i'll get back to this early tomorrow.

[lupko]

@lidavidm I have added tests (exercising numeric and array of numeric); the tests have revealed a little tweak was needed in AddPostgresTypeMetadata.

The fallback to using typename resolver because the typname is not always set. In end-to-end scenarios everything works well without the fallback because the type mapping is populated differently and includes type names.

I see some tests have failed in the CI run - checked it out and it does not look related to these changes (some Flight SQL cancellation stuff)

[lidavidm]

Thanks!