feat: Add upload_many method for multiple file uploads

[Konikz]

Description

Implements multiple file upload support by adding a new upload_many method to both Files and AsyncFiles classes. This enhancement allows users to upload multiple files in a single request, making batch operations more efficient.

Example

# Upload multiple files at once
files = client.files.upload_many(
    files=["doc1.pdf", "doc2.pdf", "doc3.pdf"]
)

# With custom configurations
files = client.files.upload_many(
    files=["doc1.pdf", "doc2.pdf"],
    configs=[
        {"display_name": "First Document"},
        {"display_name": "Second Document"}
    ]
)

Changes

• Added upload_many method to Files and AsyncFiles classes
• Implemented proper validation and error handling
• Added comprehensive docstrings with examples

Closes #293

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[janasangeetha]

Hey @Konikz
Thanks for contributing!
Kindly resolve the conflicts with the branch.