feat: add custom headers support, enhance patch_item

[mayurrawte]

Summary

• Add custom_headers parameter for server-to-server auth override
• Make x_api_key optional (can use custom_headers instead)
• Add per-request header override support
• Enhance patch_item with better documentation as the recommended way to update fields
• Add create_reference_linked_field method
• Bump version to 2.2.0

Usage

from ShipthisAPI import ShipthisAPI

# Standard usage
client = ShipthisAPI(
    organisation="your_org_id",
    x_api_key="your_api_key"
)

# Patch document fields (recommended)
client.patch_item(
    "fcl_load",
    "68a4f906743189ad061429a7",
    update_fields={"container_no": "CONT123", "seal_no": "SEAL456"}
)

Test plan

• Verify custom_headers work correctly
• Test patch_item updates fields as expected