chore: add examples with required features

[mattsse]

Motivation

makes it easier to figure out the legacy feature is required for certain examples:

cargo run -p ethers --example contract_with_abi                                                                                                                                                                           
error: target `contract_with_abi` in package `ethers` requires the features: `legacy`
Consider enabling them by passing, e.g., `--features="legacy"`

Solution

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog