Expose purser-metamask account change hook

[rdig]

While the Metamask account change hook exists and it's used inside purser-metamask (to always keep the wallet instance in sync with the UI), it's not exposed outside of the package.

This could be useful for dApps who want to act on the account being changed (Eg: log out the user, update the interface with the new instance, etc)

For this, a safer (eg: properly validated) version of the setStateEventObserver should be exposed:

purser/modules/node_modules/@colony/purser-metamask/helpers.js

Lines 133 to 157 in c4f7ca9

	/**
	* Add a new observer method to Metamask's state update events
	*
	* @method setStateEventObserver
	*
	* @param {Function} observer Function to add the state events update array
	*
	* @return {number} the length of the state events update array
	*/
	export const setStateEventObserver = (
	observer: MetamaskStateEventsObserverType,
	): void => {
	const {
	publicConfigStore: { _events: stateEvents },
	}: () => MetamaskInpageProviderType =
	/*
	* We need this little go-around trick to mock just one export of
	* the module, while leaving the rest of the module intact so we can test it
	*
	* See: https://github.com/facebook/jest/issues/936
	*/
	/* eslint-disable-next-line no-use-before-define */
	metamaskHelpers.getInpageProvider();
	return stateEvents.update.push(observer);
	};

This issue is here to track implementation of the above.

[rdig]

Note that this will need an accompanying docs.colony.io entry in order to:

• explain the reasoning behind exposing this API
• show examples on how to use in your dApp