[WIP] Get workflow history length from within workflow

[Copilot]

Working on implementing the ability to get workflow history length from within a workflow, as requested in issue #390.

Plan for Implementation

• Explore repository structure and understand current workflow execution flow
• Identify that the executor tracks lastSequenceID which represents history length
• Analyze workflow context creation and state management
• Build and test current codebase to establish baseline
• Define WorkflowInstanceInfo struct with history length and metadata
• Add workflow instance info to the workflow state/context
• Create workflow.WorkflowInstanceInfo(ctx) function to retrieve the information
• Update executor to maintain history length in workflow state
• Create tests to validate the functionality
• Test with sample workflow to verify behavior
• Update documentation if needed

The feature will allow workflows to call workflow.WorkflowInstanceInfo(ctx) to get metadata including history length, which is useful for deciding when to call ContinueAsNew.

Fixes #390.

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