JS/TS: Implement walletclient

[ashleyvega]

At the moment, we have;

• in the Console: src/contexts/wallet/ ts and tsx files
• in this repo:
  • python walletclient: https://github.com/vegaprotocol/api/blob/develop/grpc/clients/python/vegaapiclient/walletclient.py
  • (in tests) javascript clunky wallet_server_login function

    api/grpc/clients/js/test/vega/order.test.js

    Lines 10 to 30 in 18a356f

    	function wallet_server_login(walletServerURL, walletName, walletPassphrase) {
    	var request = new xhr.XMLHttpRequest();
    	var token;
    	request.onload = function () {
    	if (request.status !== 200) {
    	throw "Failed to log in to wallet server: HTTP " + request.status + " " + request.responseText;
    	}
    	var j = JSON.parse(request.responseText);
    	if (j === undefined) {
    	throw "Failed to get parse response JSON: " + request.responseText;
    	}
    	token = j.token;
    	if (token === undefined) {
    	throw "Failed to get token from response body: " + request.responseText;
    	}
    	}
    	request.open("POST", walletServerURL + "/api/v1/auth/token", true);
    	request.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
    	request.send(JSON.stringify({wallet: walletName, passphrase: walletPassphrase}, null, 2));
    	return token;
    	}

Task

Implement walletclient in Javascript/Typescript so it can be used by tests in this repo, as well as by other users of the api package (perhaps possibly the Console).