Add Signer to Api

[ianhe8x]

As a dapp, it doesn't always have access to key pair. Key pairs could be managed in a browser extension or hardware wallet. So I propose to introduce signer to api and let it do the signing. This will allow our browser extension to work with the api by exposing a signer into browser's global.

export interface Signer {
  sign(extrinsic: Extrinsic, opt: {from: string} & SignatureOptions): Promise<void>;
}

signAndSend is the place to use the signer. I prefer to changing the signature to below which looks more neat

  signAndSend (options: SignAndSendOptions)
  signAndSend (options: SignAndSendOptions, statusCb: StatusCb)
  signAndSend (options: SignAndSendOptions, statusCb?: StatusCb)

interface SignAndSendOptions extends Partial<SignatureOptions> {
  from: string | KeyringPair
}

though to keep backward compatible, it could also be like:

signAndSend (account: KeyringPair | string, _options?: Partial<SignatureOptionsPartial> | StatusCb, statusCb?: StatusCb)

I would like to hear from you if it's the right direction before submitting my PR.

[jacogr]

First going to comment on the signatures: they certainly a bit messy atm, we are not quite happy, but it is moving towards being more useful. We just always try to keep backwards compat, so happy with changes that cleans up the code as long as it does not affect downstream users.

Honestly, have not considered the signers up to now, which is certainly a gap. (The apps UI manages this by putting stuff on a queue when sent - which is not great) So happy to look at any suggestions in this area, it is certainly something that needs to be addressed.

We should probably keep the signature as you suggested as a last point - would just allow account: KeyringPair | string | AccountId | Address as well, the latter 2 can just be toString-ed. (Technically we need AccountIndex support as well, which is also part of Address - but for v1, can forgo that and the lookup conversion)

TL;DR good idea, we certainly need support sooner rather than later

EDIT: Re-wrote since some stuff (commented on via phone, was a bit less than sensical)

[jacogr]

Closed in #662

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