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 refactoring the handling of ethers.Wallet and related types in the codebase. It replaces the custom EthersWallet type with a more generic ValidSigner type, streamlining the code and enhancing type safety.

Detailed summary

• Removed custom type EthersWallet and replaced it with ValidSigner.
• Updated tests to reflect the removal of EthersWallet.
• Adjusted function signatures in toSigner to accept ValidSigner.
• Added support for JsonRpcSigner in tests.
• Cleaned up unnecessary type imports and code comments.

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[github-actions]

size-limit report 📦

Path	Size
core (esm)	14.93 KB (+0.29% 🔺)
core (cjs)	20.99 KB (+0.11% 🔺)
bundler (tree-shaking)	544 B (0%)
paymaster (tree-shaking)	543 B (0%)