Create a new TaxRateConfiguration

The tax_authority field is only accepted on this endpoint for Canadian accounts with the tax_authority feature enabled.

When available, tax_authority is required — omitting it returns 422 with the message "tax_authority is required for Canadian accounts with tax authority support enabled."

When not available (non-Canadian account, or Canadian account without the feature enabled), supplying tax_authority returns 422 with the message "tax_authority is only available for Canadian accounts with tax authority support enabled."

tax_authority is a create-only field and is ignored on PATCH.