[Justin Hammond (Justintime50)]

Description

Exposes the new EU regulatory API fields in the C# client library for DOCS-947 and bumps the version to v7.12.0.

• Adds ManufacturerProductId (manufacturer_product_id, NS-PID) and StandardizedProductId (standardized_product_id, S-PID such as a GTIN/EAN) to the CustomsItem API model and the CustomsItem.Create parameters. The create parameters work for standalone customs item creation and nested customs items in CustomsInfo.Create.
• Adds Address to the TaxIdentifier API model and the TaxIdentifier.Create parameters. The parameter takes a Models.API.Address, so you can pass either a reference (new Address { Id = "adr_..." }) or inline address fields. Parameters/Address/Create.cs is unchanged.
• UPS carrier_options (ups.shipper_type / ups.consignee_type) needs no change. It can already be sent through Options.AddAdditionalOption("carrier_options", ...).
• Bumps the version to v7.12.0 in EasyPost.nuspec and VersionInfo.cs, and adds the CHANGELOG entry.

Testing

No new tests or cassettes. These are additive optional properties that only expose existing API fields, so this follows the #667 precedent. The library builds locally (just build-fw net10.0, plus netstandard2.0) and the dotnet-format check passes. CI covers the rest.

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)