Improve implementation selection strategy

[jeluard]

Usage of bls is problematic under some alternative JS environment for the following reasons:

• usage of top-level-wait in a lib
• automatic detection of the proper implementation (i.e. use native when node env is detected)

Ideally there would be a way to allow bls users to select the implementation used at runtime. Users should not be able to specify their chosen implementation after the initialization has been done (thus ignoring their choice).

First idea

Single global implementation

BLS_IMPLEMENTATION=herumi node ...

import bls, {init} from "@chainsafe/bls";

// Make sure the initialization has been done
// Can be called several times, initialization is done only once
await init();

const secretKey = bls.SecretKey.fromKeygen();
const message = new Uint8Array(32);
const signature = secretKey.sign(message);

No automatic init would be done, users explicitly have to do it. Essentially it's a modified version (with env variable) of the existing switchable singleton. This would be the only way to use the lib.

Second idea

Explicitly pass around a bls object tied to an implementation.

const bls = await BLS.init("herumi");
const secretKey = bls.SecretKey.fromKeygen();
const message = new Uint8Array(32);
const signature = secretKey.sign(message);