Add support for file metadata and signed URL generation in Gofr GCS file store

[NitinKumar004]

Description:

We use Gofr's GCS file store to upload files. Gofr currently supports basic file uploads, but we need:

1. File metadata support:

   • Set Content-Type (e.g., "text/csv") — tells the browser how to handle the file
   • Set Content-Disposition (e.g., "attachment; filename='report.csv'") — forces download with a specific filename

   These metadata fields are needed so browsers download files correctly instead of displaying them.

2. Signed URL generation:

   • Generate signed URLs for uploaded files with configurable expiry time

   This creates time-limited, secure download links that can be shared via email. The URL expires after the set duration, providing secure access without making files public.

These features are supported by GCS, but Gofr's file store doesn't expose them. We currently use the GCS SDK directly, which bypasses Gofr's abstraction.