Add dedicated credential property to BigQuery metastore catalog

### Feature Request / Improvement

This is a follow-up of https://github.com/apache/iceberg/pull/12808.

The PR uses the `GoogleCredentials.getApplicationDefault()` method to create the BigQuery client. The approach doesn't work well when we want to access multiple BigQuery projects from a single machine.

I recommend adding a new property reading JSON key or file with the `GoogleCredentials.fromStream` method.

### Query engine

Starburst

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start by reading the BigQuery metastore catalog code introduced by PR 12808 and trace its use of GoogleCredentials.getApplicationDefault(). Define the credential property behavior around GoogleCredentials.fromStream for a JSON key or file, then add coverage showing separate BigQuery projects can use separate credentials.

Written by the indexing model from the issue text.