feat: improve Google Drive credential flexibility and readability

[rmanibus]

Problem

The Google Drive source (gdrive.go) currently only accepts credentials via file paths or secret refs. This limits library users who manage auth externally and CI/CD environments where mounting credential files is inconvenient.

Additionally, the auth logic in NewGDriveSource is a 60-line nested if/else chain — hard to read and extend.

Proposed Solution

Library-level (pkg/source)

• WithDriveService(srv) — inject a pre-built *drive.Service, skipping all internal auth (escape hatch for ADC, custom TokenSource, etc.)
• WithCredsJSON(data) — provide credentials as inline JSON bytes (OAuth client or service account), mirroring rclone's service_account_credentials
• Refactor the auth if/else chain into focused helpers: buildDriveService, loadCredsBytes, serviceFromCredsBytes, resolveDriveName

CLI-level (cmd/cloudstic)

• --google-credentials-json flag + GOOGLE_CREDENTIALS_JSON env var
• Wire through profiles (ProfileAuth.GoogleCredsJSON, BackupProfile.GoogleCredsJSON)

Documentation

• Update user guide with credential priorities, usage examples, and env var table

Comparison with Other Tools

Tool	Inline JSON support
rclone	✅ service_account_credentials
duplicacy	❌ file only
cloudstic (after this)	✅ --google-credentials-json / GOOGLE_CREDENTIALS_JSON