fix(modelgateway): Number of partitions retrieval

[MiguelAAe]

Why

Issues

When the config from kafka numPartitions is not present or isn't a string the model gateway would fail to create an infer consumer due to the casting of an integer or an empty value from the Topics config.

Motivation

What

Summary of changes

• update the kafka config logic retrieval of topics to check for either strings or integer and positive integer
• updated error messages when erroneous config is loaded

Checklist

• Added/updated unit tests
• Added/updated documentation
• Checked for typos in variable names, comments, etc.
• Added licences for new files

Testing

[lc525]

lgtm