Add ByPropertyNamed grouping rule for partitioned messaging

[jeremydmiller]

Summary

• Adds a new built-in IGroupingRule (PropertyNameGroupingRule) that determines the GroupId by looking for a named property on the message type at runtime
• Adds ByPropertyNamed(params string[] propertyNames) to the MessagePartitioningRules fluent API
• Property accessors are compiled via LambdaBuilder and memoized per message type using ImHashMap for performance
• Supports any property type (string, int, Guid, long, etc.) via ToString(), with null values becoming string.Empty
• Particularly useful when message contracts are auto-generated (e.g. from .proto files) and cannot implement a marker interface

Closes #2228

Test plan

• 11 unit tests covering string, int, Guid, and long property types
• Tests for null property values, missing properties, property name priority, memoization, and explicit GroupId precedence
• All 28 existing partitioning tests continue to pass
• Documentation sample compiles successfully

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