Add wallet_openDeepLink rpc method

[hmalik88]

This PR aims to add a specification for the wallet_openDeepLink method. The motivation behind this adding this functionality is to make use of MetaMask URLs to be able to programmatically navigate to different parts of our clients and snaps.

[vandan]

Sounds like we should add some information about the expected return values in here.

[vandan]

Would we want to enable a flow where:
dapp --deeplinks to--> MetaMask internal URL -> some action is completed --MetaMask redirects to --> dapp URL

Essentially supporting a "return URL" similar to the behavior at the end of onboarding when wallet_registerOnboarding is used to return the user to the dapp that initiated wallet onboarding.

[hmalik88]

I think the wallet onboarding is a unique flow, I can't think of other use cases where deep links kick you back somewhere. We would have to keep state of the ongoing actions and track when they're completed, which just completely blows the scope of this feature and not really in line with a simple deep link imo. I'd like to keep this as simple as possible.