chore: add paymaster

[joepegler]

PR-Codex overview

This PR updates the paymaster functionality by introducing a new PAYMASTER_URL environment variable and refactoring the Paymaster class.

Detailed summary

• Added PAYMASTER_URL environment variable
• Renamed BiconomyPaymaster class to Paymaster
• Updated Paymaster class methods and properties
• Refactored type definitions for paymaster modes

The following files were skipped due to too many changes: src/account/NexusSmartAccount.ts

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[github-actions]

size-limit report 📦

Path	Size
core (esm)	15.8 KB (0%)
core (cjs)	20.93 KB (0%)
account (tree-shaking)	13.66 KB (0%)
bundler (tree-shaking)	4.6 KB (0%)
paymaster (tree-shaking)	2.42 KB (0%)