Add APDB schema definition based on felis (DM-34480)

This schema is the same as currently defined in existing YAML files in
dax_apdb and ap_association packages. Some columns have their `nullable`
attributes corrected to reflect the design. There are differences w.r.t.
baselineSchema, in particular APDB schema does not include few columns
that were added recently to baseline schema - `nearbyExtObj[1-3]`,
`nearbyLowzGal`, and corresponding separation columns. And there are few
columns in APDB that do not exist in baseline schema for DIA tables.