Add VALIDATA_SCHEMAS_CONFIG env var support
This is needed to fix validata-api#1 (closed)
It introduces a
VALIDATA_SCHEMAS_CONFIG environment variable that can point to an alternative
schemas.toml file path. This removes the need for the
--config CLI option IMHO.
It also removes the test for the
scdl- prefix on schema keys, because alternative schema producers will want to use different prefixes.