Add vendor extension support to Content.Encoding type

[ayushshrivastv]

Issue #315

This PR adds vendor extension support to the Content.Encoding type, addressing issue #315. The implementation follows the existing patterns used throughout the codebase for vendor extensions.

Changes made:

• Added VendorExtendable protocol conformance to Content.Encoding
• Introduced vendorExtensions property to store custom extensions
• Updated initializers to support optional vendor extensions
• Modified CodingKeys to properly handle vendor extension encoding/decoding
• Added comprehensive test cases for both encoding and decoding of vendor extensions

The changes are non-breaking as they:

• Maintain backward compatibility with existing code
• Make vendorExtensions parameter optional with a default empty dictionary
• Preserve all existing functionality

I've implemented these changes in both OpenAPIKit and OpenAPIKit30 to maintain consistency across versions. All tests are passing, including the new vendor extension test cases.

This enhancement allows users to add custom vendor extensions to Content.Encoding objects, following the OpenAPI specification's vendor extension pattern (x-prefixed properties).

[mattpolzin]

Thanks!