Allow custom schema and data file paths, print file path on registering

[m-dwyer]

Context

We currently provide an Avro schema file and sample data JSON that will allow users to run devbox run populate, resulting in Avro-encoded payloads being published to the outbox table.

However, the environment variables for these are hard-coded, and cannot be overridden

Changes

• Set relevant environment variables as export in init_hook, so users can override and register their own schema plus publish their own data
• Print out the schema file path when registering against schema registry