Extend Network feature to model service sessions

## Motivation  

- Currently only interactive sessions are capable of attaching project-wide networks upon startup.

## Objective  

- Make model service (inference) sessions also able to attach project-wide networks.

## Details  

1. Create a new DB column called `network` which references the `NetworkRow` table
1. Update model service CRUD APIs to be able to manipulate the `network` field
1. Upon model service session creation (`handle_route_creation()` handler inside registry.py) provide the `network` value to `create_session()` internal API
1. Update network removal API to block action when there are active model services referencing the target network

## Impact    

-

JIRA Issue: BA-919

Start with NetworkRow and the model service CRUD APIs, then inspect handle_route_creation() in registry.py and the create_session() internal API. Trace the network removal API and identify how active model services reference a network. Done means model services can create, read, update, and delete their network value, session creation passes it through, and removal is blocked while it is referenced.

Written by the indexing model from the issue text.