[simplex]: Investigate paymaster support

[seunlanlege]

I think long term, we should not mandate that fillers hold the native tokens of all the chains they want to support. They will already hold USDC/USDT. This also simplifies the filler, so we don't have to manage entrypoint deposits

We should investigate a bundler-agnostic mechanism to support USDC/USDT paymasters.

[seunlanlege]

After reading the spec a bit more, I'd prefer we leverage a permissionless, fully onchain paymaster. (No API keys)

If none exist, running a paymaster ourselves is quite trivial.

[seunlanlege]

Until USDT supports EIP-2612 I think circle paymaster + USDC is good enough