Introduce Provider and Signer

[janek26]

The API of the starknet package got a bit out of hand with more and more utility and types.
closes #49

Inspired by ethers I introduced the concept of providers and signers, which can also get passed into more high level abstractions, such as the Contract class.

Provider

To keep it simple (and because there's currently just one network goerli-alpha3) it also exports a defaultProvider from the very top level. So you can still getting started by doing

import { defaultProvider } from 'starknet';

defaultProvider.getContractAddresses().then((data) => {
  console.log(data);
});

using a provider (normally providers are read only, but as you can write to starknet without signing, providers can also write to starknet using senderAddress = 0) you can specify the network you wanna use, in preparation for main net launch.

import { Provider } from 'starknet';

// not supported right now, as main net isn't online yet
const mainnetProvider = new Provider({ network: "mainnet" });

mainnetProvider.getContractAddresses().then((data) => {
  console.log(data);
});

Signer

Signer can be used to intercept specific methods from a provider, like addTransaction to add a signature to every call. This is useful to interact with standard wallet implementations on starknet, like open zeppelin and argent.
Signers can be provided by ie an extension, and their async methods can require user interactions with the extension before they resolve. A very simple Signer is shipped with starknet.js:

import { Signer, defaultProvider, ec } from 'starknet';

// random key pair 
const starkKeyPair = ec.genKeyPair();

// sign every request with a local key pair and invoke it through an `execute` endpoint on the given address (default wallet implementation)
const localSigner = new Signer(defaultProvider, WALLET_ADDRESS , starkKeyPair);

localSigner.getContractAddresses().then((data) => {
  console.log(data);
  // the signer could intercept this function call and append some signer specific logic
  localSigner.invokeFunction(contractAddress, entrypoint, calldata).then(response => {
    console.log("invokeResult:", response);
  });
});

As a result this PR changes the way many functions are exported, therefore this is considered a BREAKING CHANGE and MAJOR version bump.

[0xs34n]

Hey @janek26 , at first glance I really love this implementation. I think it strikes a great balance between standard defaults and also flexibility for developers. I will play around with it (hopefully this weekend) and let you know my feedback and merge it. Thanks!

[0xs34n]

Thanks, @delaaxe for reviewing! @janek26 feel free to merge to dev when it's ready.

[github-actions]

🎉 This PR is included in version 1.6.0 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

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