Add Metamask Snap integration

[BeroBurny]

Integrate Metamask Snaps Accounts into Polkadot JS Apps

[danforbes]

@jacogr is there anything else you need from us before running these workflows? This work was done as part of a W3F grant and it would be great to get it out there for the community!

[takahser]

@BeroBurny @danforbes thanks for your PR which is part of this delivery.
I added some comments, feel free to check them out.

[takahser]

1. Why does the return time have to be Promise<boolean[]>? After all, initPolkadotSnap is just returning a single Promise<boolean>.

2. In general, this code could be simplified when using rxjs which already is a dependency:

Suggested change

	export function injectExtensions (): Promise<boolean[]> {
	return Promise.all([
	initPolkadotSnap
	].map((method) => {
	// Timeout injecting extension
	return Promise.race([
	method(),
	new Promise<false>((resolve) => {
	setTimeout((): void => {
	resolve(false);
	}, 1000 /* 1 sec */);
	})
	]);
	}));
	}
	export const injectExtensions = () =>
	firstValueFrom(from(initPolkadotSnap())
	.pipe(
	timeout(1000), // timeout if initPolkadotSnap() doesn't resolve after 1s
	catchError(() => of(false)),
	));

(Please note that I haven't actually tested this code)

[BeroBurny]

1. Idea is to make it future proof it there will be another wallet like MM snaps so users can easily add it to apps
   for example @polkadot/extension-compat-metamask you can just import initMetaMask and add it to the list to support it

2. did the same solution with RxJs (personally for me is more complex as I'm uset to async/await js)

[semuelle]

@BeroBurny / @danforbes, could you resolve the conflicts or resubmit, so this has a chance of being merged?

[danforbes]

Thank you for the feedback, @semuelle! I have informed @BeroBurny and @irubido that this Issue should take priority over the other one you recently commented on - please let me know if you disagree 🙏🏻

[danforbes]

@semuelle - this PR should be up-to-date 🚀 Please let us know if there's anything else we can do to help the team evaluate things. We're getting started on support for parachains, etc now 💪🏻