fix: signer typescript issues

[VGabriel45]

This PR fixes the typescript issues related to other unknown signer types.
This PR specifically fixes the issues mentioned in this ticket: https://linear.app/biconomy-20/issue/BIC-104/integration-issue-privy-embedded-wallet-with-biconomy-nexus-sdk

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PR-Codex overview

This PR focuses on refining the handling of EthersWallet and introducing a new type ValidSigner to enhance type safety. It simplifies the code by removing unnecessary type assertions and improves the toSigner function to accommodate various signer types.

Detailed summary

• Removed EthersWallet type and replaced it with ValidSigner.
• Updated toSigner function to use ValidSigner for better type handling.
• Added tests for ethers.JsonRpcSigner support.
• Cleaned up imports and type definitions across several files.
• Simplified wallet creation in tests by removing unnecessary type assertions.

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[github-actions]

size-limit report 📦

Path	Size
core (esm)	16.3 KB (-0.14% 🔽)
core (cjs)	22.83 KB (+0.26% 🔺)
bundler (tree-shaking)	518 B (0%)
paymaster (tree-shaking)	543 B (0%)