#5257 Increase default gas limit for Ethereum (gateway)

• Move gasLimit config from connectors to the chains
• Set the default gas limit to 3,000,000. It's a lot, but using uniswap v3 is expensive. We want it to work by default for users that choose uniswap v3.

Link: https://github.com/hummingbot/hummingbot/pull/5257