Change 'custom schema' example to use 'extensions and schemas' terminology

[dixonjoel]

https://github.com/ni/datastore-python/tree/main/examples/notebooks/custom-metadata was ported from another repo. The newest terminology is not custom metadata or custom schemas, but rather 'extensions' that work together with the 'schema_id'.
.proto file

[dixonjoel]

Fixed by #75