feat: add LoadIncrementalFilteredPolicy() API for incremental filtered policy loading

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Fix: #387

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[hsluoyz]

@copilot for code and test code, need to refer to Golang Casbin as much as possible: casbin/casbin#582

[Copilot]

@copilot for code and test code, need to refer to Golang Casbin as much as possible: casbin/casbin#582

Refactored to match the Golang pattern from PR #582 (commit e7e7989):

Key changes to align with Golang architecture:

• Removed separate adapter methods (LoadIncrementalFilteredPolicy from IFilteredAdapter)
• Created shared internal method LoadFilteredPolicyInternal (similar to Golang's loadFilteredPolicy)
• LoadFilteredPolicy now calls ClearPolicy() before loading (matches Golang's model.ClearPolicy())
• LoadIncrementalFilteredPolicy directly calls the shared method without clearing
• Both methods now use the same underlying logic, differing only in whether they clear first

This matches the pattern where Golang has one adapter method and the clearing happens at the enforcer/model level, not in the adapter.

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