Add transaction signing capabilities

[alisina-nl]

This PR adds the following features to Ethers:

• A Signer behaviour which has mandatory callbacks for listing accounts and signing transactions.
• Two built-in signer implementations:
  • Signer.Local: A local signer which signs transactions and lists accounts (a single account) using a 256bit private key.
  • Signer.JsonRPC: A signer which delegates the transaction signing to a RPC server (like web3siger or geth) using the eth_signTransaction and eth_accounts functions.
• Transaction struct with support for encoding EIP-1559 and Legacy transactions.
• A new Ethers.sign_transaction/2 function which returns signed payload of a transaction.
• Accepting signer and signer_opts options in Ethers.send/2 and Ethers.deploy/2 functions.
• Updated documentation (+ README.md) with the new signer.

Closes fortelabsinc#1

[wchenNL]

while testing with this branch found a few very minor things which may not be related to this PR:

• default_address is required otherwise a {:error, :no_to_address} is returned, so I guess it's not really optional?
• extra comma in the readme at this line
  default_signer_opts: [] # Defaults to: []

[alisinabh]

• default_address is required otherwise a {:error, :no_to_address} is returned, so I guess it's not really optional?

This is true. If you don't provide the default address you MUST specify to address when calling or sending a transaction. In that case you can do it like this (in the second argument of send/call function):

MyERC20Token.name()
|> Ethers.call(to: "0xContract Address Here")

If this is not well communicated in the docs maybe we should improve them.

[wchenNL]

LGTM!