Adds activity host registration support

[sfmskywalker]

Adds support for registering activity hosts, enabling code-first activity generation from CLR types.

• Introduces HostMethodActivity to execute public async methods on CLR types.
• Implements HostMethodActivityProvider to provide activity descriptors for configured host methods.
• Creates IHostMethodActivityDescriber to describe host method activities.
• Adds FromServicesAttribute to resolve host method parameters from the service provider.
• Updates BookmarkExecutionContextExtensions and the Bookmark model to improve bookmark management logic and structure.

This allows developers to register classes whose methods will be automatically exposed as activities in the workflow designer.

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This change is 

[Copilot]

Pull request overview

This PR introduces a code-first activity generation system that allows developers to register CLR types whose public methods are automatically exposed as workflow activities. The main addition is the HostMethodActivity infrastructure that dynamically invokes methods on host types and manages their inputs/outputs.

• Adds HostMethodActivity as the core runtime activity that executes methods on registered host types
• Implements activity descriptor generation via HostMethodActivityDescriber and HostMethodActivityProvider
• Converts Bookmark from immutable record to mutable class to support callback metadata modification
• Refactors BookmarkExecutionContextExtensions to use C# 13.0 extension member syntax

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
BackgroundActivityInvokerMiddleware.cs	Changes default to null for nullable delegate assignments for better clarity
HostMethodActivityDescriber.cs	New service that generates activity descriptors from host type methods using reflection
DelegateHostMethodParameterValueProvider.cs	New convenience wrapper for custom parameter value resolution logic
DefaultHostMethodParameterValueProvider.cs	New default implementation for resolving method parameters from workflow inputs or DI
DefaultExpressionDescriptorProvider.cs	Removes unused System.Diagnostics using statement
HostMethodActivitiesOptions.cs	New options class for registering host types to expose as activities
WorkflowManagementFeature.cs	Adds AddActivityHost methods and registers host method services
CachingWorkflowDefinitionsFeature.cs	Removes unused Elsa.Workflows.Management.Handlers using statement
WorkflowInstanceStoreExtensions.cs	Removes unused Elsa.Workflows.Management.Filters using statement
ModuleExtensions.cs	Adds convenience AddActivityHost extension method for module configuration
IHostMethodParameterValueProvider.cs	New interface and context types for extensible parameter value resolution
IHostMethodActivityDescriber.cs	New interface for describing host method activities
FromServicesAttribute.cs	New attribute to mark parameters for DI resolution instead of workflow inputs
HostMethodActivityProvider.cs	New activity provider that exposes host type methods as workflow activities
HostMethodActivity.cs	New runtime activity that invokes host type methods and manages bookmark callbacks
Bookmark.cs	Converts from immutable record to mutable class with init properties to allow metadata modification
InputAttribute.cs	Extends targets to include Method and ReturnValue for host method scenarios
ActivityAttribute.cs	Extends targets to include Struct and Interface for broader host type support
BookmarkExecutionContextExtensions.cs	Refactors to use C# 13.0 extension member syntax for better organization