Add `Type` to `Keyring` as generic type parameter

[mikesposito]

This PR adds the Type extends string generic type parameter to Keyring, making it Keyring<Type, String>. This will help to infer types coming from builders in KeyringController. In that regard, this PR also adds a KeyringBuilder type.

• Related to https://github.com/MetaMask/core/issues/4366

[mikesposito]

Converting to draft until I have another draft PR on KeyringController to show how this would help

[MajorLift]

Nit: Would it make sense to use Kind (i.e. type of type) instead of Type here as the name of the generic parameter (and "kind" instead of "type" as the type property name), since we're trying to specify sub-categories of the Keyring type?

The term "type" is generally used for categories of values not types. Here's an example of "kind" being used in the TypeScript documentation.

[mikesposito]

The Keyring type has been moved to the @metamask/keyring-utils package, and has been deprecated here in utils. I'm closing this and will reopen it in the new repo.