Tracking snap_manageAccounts permission

[tmpfs]

This issue is to track the tasks required to land the draft version of the snap_manageAccounts permission implementation.

• Peer review the draft snap-keyring implementation, see https://github.com/LavaMoat/snap-keyring/
• Merge the PR which adds the implementation to @metamask/rpc-methods, see Add snap_manageAccounts RPC handler. #549
• Merge the PR which adds the snap-keyring to the keyringController and activates the snap_manageAccounts method implementation, see Manage accounts feature for snaps metamask-extension#14967

In addition we would need to decide on whether the isolation for the managed accounts is acceptable. Currently, accounts are isolated by snap identifier which means that private data is accessible from multiple dapps; for my use case implementing the threshold signatures snap this is not the correct behavior and we should work out a strategy for allowing multiple isolation scopes (snap or dapp). For some more discussion about this, see #548.

Related to SSIP-2.

/cc @rekmarks, @ritave

[gantunesr]

This is already completed and the snap_manageAccounts is already available in both stable and Flask