Workflow Definition Picker

[sfmskywalker]

=== auto-pr-body ===

Pull Request Body:
This pull request features the following changes:

Summary

• Added a protected async method OnAfterRenderAsync to retrieve and store available workflow definitions in a _items variable
• Added a private method, GetSelectedValue, to retrieve the literal value for a given variable
• Added a private method, OnValueChanged, to update the expression when a value is changed
• Updated ServiceCollection extensions class to add a new UIHintHandler for workflow definition picking
• Added a new UIHintHandler class: WorkflowDefinitionPickerHandler
• Updated VariablePickerHandler to use the WellKnownSyntaxNames.Literal syntax

List of Changes

• Added a YAML workflow file: .github\workflows\pr-body-generator.yml
• Added the YAML workflow path to Elsa.Studio.sln
• Extended IUIHintHandler interface with new members in src/core/Elsa.Studio.Core/Contracts/IUIHintHandler.cs
• Added a component for picking a variable in src/modules/Elsa.Studio.UIHintHandlers/Components/VariablePicker.razor.cs
• Added a component for picking a workflow definition in src/modules/Elsa.Studio.UIHintHandlers/Components/WorkflowDefinitionPicker.razor and src/modules/Elsa.Studio.UIHintHandlersp/Components/WorkflowDefinitionPicker.razor.cs

Refactoring Target

• Refactor YAML workflow file for improved readability
• Rename components to reflect more clearly what they do and to allow developers to quickly assess what they are for
• Move the _items variable to a class-level variable and make it private
• Move the GetSelectedValue and OnValueChanged methods to the WorkflowDefinitionPickerHandler class to follow principles of separation of concerns
• In the VariablePickerHandler, move the UISyntax to the constuctor and assign the value once for more efficient access
• Increase code coverage with automated tests for new components and for the extended interface.