Support dapp origin identifier in methodImplementation context

[tmpfs]

Following on from the conversation in #548.

If we take the snap_manangeAccounts feature that I am implementing for the threshold signatures snap as an example, I think it would be correct if the accounts it manages were not available to other dapps. Right now, this isn't possible as isolation is achieved using the snapId exposed via context.origin.

This permission is sensitive (like getBIP44Entropy_*) in that it allows access to private key information so in this instance it should be isolated to the owner dapp.

Would it be possible to expose the dapp origin on the context object so we could achieve this? As origin already is the snapId possibly we call it originString like the argument in the snap RPC handler for the moment? Although to prevent confusion, I think it would be good in the long run to rename context.origin to context.snapId and point context.origin to the dapp origin if possible.

/cc @rekmarks

[gantunesr]

This is no longer relevant since we do a verification using the allowedOrigins in the endowments