Web3 signer functionality (#56)

This PR branches from
[lambdaclass:signer](https://github.com/lambdaclass/eigen-rs/tree/signer)
branch

This PR adds web3 signer functionality. At the moment we have
implemented:
* A function to retrieve a remote signer from AWS Key Management Service
* A web3 signer that gets a transaction signature by sending an RPC
request

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Co-authored-by: tomasarrachea <tomas.arrachea@lambdaclass.com>