Release v2.2.0

[ianhalpern]

Description

This PR adds new SDK features, improves internal architecture, and fixes bugs in preparation for the next release.

Changes include:

• Add static All/AllAsync methods to ARMObjectBase for querying all objects of a type
• Add static GroupBy and OrderBy methods to ARMObjectBase for query building
• Add InvoiceAttachment ARMObject class and Session accessor
• Move _httpMessageHandler from ARMRequest to Payload.Session for cleaner test mocking
• Extract BuildRequest/ExtractArgs private helpers in ARMObjectBase to eliminate duplicated session extraction and polymorphic filter logic across FilterBy, Select, GroupBy, OrderBy, and AllAsync
• Add polymorphic filter to Select, GroupBy, and OrderBy (previously only applied in FilterBy)
• Fix GetRoute pluralization to avoid double-s on objects ending in "s" (e.g. processing_settings)
• Fix ARMRequest.GroupBy bug where passing a string would throw InvalidCastException (string implements IEnumerable)
• Rename Session constructor params from api_key/api_version to apiKey/apiVersion (C# naming convention)
• Extract exception classes to Payload.Exceptions namespace with backward-compatible aliases in pl
• Add TestARMObjectStatic test suite with 7 mock-based tests covering All, GroupBy, OrderBy, Select, and polymorphic filter propagation
• Update TestApiVersioning tests to use Session._httpMessageHandler instead of ARMRequest._httpMessageHandler
• Clean up duplicate/unused imports in TestSession.cs
• Fix publish.yml workflow name from "Publish Gem" to "Publish Nuget Package"

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which enhances functionality)
• This change requires a documentation update