Add account types

[danfinlay]

Introduces a concept of "account types" to the AccountsController.

The permission to wallet_manageIdentities is now known as wallet_manageAccounts_*, where * is an account type identifier string.

Currently this string is Ether for Ethereum accounts, per Ethereum's SLIP44 name.

Also introduces wallet_accounts_*, where * is again that same account type identifier. This method is meant to so that eth_accounts can be an alias for wallet_accounts_Ether, as a strict superset of that method.

If any parameters are present in a call to wallet_accounts_*, then those parameters will be passed to the handler for the account defined by the from value on its AccountMessageHandler function, allowing account type snaps to define their own interfaces, and the accounts permission to allow passing through messages related to the permitted accounts.

Will benefit from a number of improvements:

• [ ] Rendering protocol names from slip44 identifier.
• [ ] Allow selecting (attenuating) accounts of non-eth protocols.
• [ ] Redirect eth_accounts at wallet_accounts_*.
• [ ] See if we can redirect all classic eth signing methods through this new general purpose send method (allows keyrings to iterate faster than the keyring-controller).

[danfinlay]

Alright got the tests passing.

I separated the two types of identifiers:

• derivation identifiers, like slip44:60
• account type identifiers, like Ether

We are using the former in Aaron's PR above to ensure that the key derivation permission is bound very explicitly to the input of the key derivation function.

The account type identifier is a "softer" guarantee: The two permissions that use this are for:

• Registering accounts, and handling their incoming messages
• Connecting to accounts, and sending them incoming messages.

To allow keyless account types, it's important that the account-service registry is not bound to a key derivation strategy (or key derivation at all, as in the case of contract accounts).