Add an optional keyFilename flag to specify credentials file

[digitalWestie]

This PR introduces a -k or --keyFilename flag so that you can use a service account key file instead of using the Gcloud CLI to authenticate when using the command line runners.

E.g.

npx ts-node ./library/runner-cli/categorization_runner.ts --vertexProject "myproject-466109" --outputFile glasgow-categorized.csv --inputFile ./../glasgowslaverycsv.csv --additionalContext "Comments about Glasgow's historical connections to slavery and how the city should address this legacy" --keyFilename /path/to/account-key.json