Gen-schema-view: option to include old data as part of the schema builder.

### Extension name

firestore-bigquery-export

### What feature would you like to see?

I would like to include old_data in the gen-schema-view script?

### How would you use it?

When generating a new schema, I would like to include old_data as part of the schema data.

Start by locating the gen-schema-view script in the firestore-bigquery-export extension and tracing which data fields it uses to build a schema. Check how old_data is represented and whether related tests or fixtures exist. Done means the schema-generation option includes old_data when requested and existing behavior remains unchanged otherwise.

Written by the indexing model from the issue text.