[go] Replace caigo with gnark-crypto for signing

[amayvs]

• Fixes garbage collection issues observed for large amount of orders
• GnarkSign improves signing speed by 4x when compared to caigo.Curve.Sign
• Add new method to generate GetEcdsaPrivateKey from hex string
• Cleanup methods and benchmarks used in signable_starknet.go

[amayvs]

@sv Benchmarks and comparison test already using Gnark to sign via random private key, no update required there

There wasn’t a clear way to use private key string earlier, now updated/added in onboarding example