feat: improve file validation performance

[rjanjua]

adds:

• improved performance of file/meta validation significantly (100k files with metadata takes seconds rather than 10+ minutes)
• consolidated all file validation into one place
• fixed nesting for meta files that are nested in separate directories to their raw counterparts #33
• simplified logic for finding all files given in passed file paths
• added 'recursive' arg to upload function

[github-actions[bot]]

Coverage report

The coverage rate went from 95% to 96% :arrow_up: The branch rate is 87%.

100% of new lines are covered.

Diff Coverage details (click to unfold)

### deepset_cloud_sdk/s3/upload.py `100%` of new lines are covered (`96%` of the complete file). ### deepset_cloud_sdk/api/upload_sessions.py `100%` of new lines are covered (`100%` of the complete file). ### deepset_cloud_sdk/service/files_service.py `100%` of new lines are covered (`96%` of the complete file). ### deepset_cloud_sdk/api/files.py `100%` of new lines are covered (`91%` of the complete file).