Create sign/ecdsa package - Second step of crypto refactoring

[devin-ai-integration]

Update sign/ecdsa package to use common.Overwrite function

Summary

This PR addresses a GitHub comment from @tarakby requesting consolidation of the overwrite function usage across the codebase. The changes update the sign/ecdsa package to use the centralized common.Overwrite function instead of maintaining a local overwrite function.

Changes made:

• Added import for github.com/onflow/crypto/common in sign/ecdsa/ecdsa.go
• Removed local overwrite function definition (4 lines)
• Updated single usage from defer overwrite(okm) to defer common.Overwrite(okm) in ECDSA key generation

This follows the same pattern already established in bls.go and consolidates memory clearing functionality into a single, more secure implementation.

Review & Testing Checklist for Human

• Verify ECDSA key generation and signing still works correctly - Test both P-256 and secp256k1 curves to ensure no regression in cryptographic functionality
• Confirm memory security properties are maintained - The new common.Overwrite uses crypto/rand.Read() before zero-fill, which is more secure than the old simple zero-fill loop
• Test end-to-end ECDSA workflows - Generate keys, sign messages, verify signatures to ensure no performance or correctness regressions

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end
    
    subgraph "Crypto Package Structure"
        A["sign/ecdsa/ecdsa.go"]:::major-edit
        B["common/common.go"]:::context
        C["bls.go"]:::context
    end
    
    A -->|"imports & uses"| B
    C -->|"already uses"| B
    
    B -->|"provides Overwrite()"| D["Secure Memory Clearing"]
    A -->|"key generation"| E["ECDSA Private Keys"]
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB  
    classDef context fill:#FFFFFF

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Notes

• This change is part of the larger crypto package refactoring effort to create modular, well-structured packages
• The common.Overwrite function is actually more secure than the old local version as it uses cryptographically secure randomization before zero-fill
• All CI checks passed (Unit Tests, Go code checks, C code checks)
• Link to Devin run: https://app.devin.ai/sessions/5cc7e1e8f7f0401999ebefe0c7610ca5
• Requested by: @tarakby

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