feat: Add Tool Memory type with specialized metadata and statistics

[Nsuccess]

Closes #243

• Add ToolCallResult model for tracking tool execution history
• Add 'tool' to MemoryType literal
• Add when_to_use field for retrieval hints
• Add metadata field for type-specific data
• Add tool_calls field for tool execution history
• Add get_tool_statistics() method for success_rate, avg_time_cost, avg_score
• Add tool.py prompt for tool memory extraction
• Update repository interfaces and implementations (inmemory, postgres)
• Add 14 unit tests for Tool Memory feature

[evan-ak]

Thank you very much for your contribution. We are actively working on merging this feature into MemU.

Given that there are multiple PRs introducing various kinds of fields to the memory item, and that some of them are exclusively used by specific memory types or applications, we’ve decided to consolidate these attributes into a single "extra" field to keep the memory item’s structure clear and concise.

As a result, we are still working on a few minor revisions to the current commits and will proceed with the merge once these adjustments are complete.

Thanks again for the great work!