refactor!: LedgerKeyring implements Keyring type

[mikesposito]

The LedgerKeyring is being overhauled to implement the Keyring type from @metamask/utils. Most of the changes are due to the addresses type that is now Hex

### Changed

- **BREAKING:** `LedgerKeyring` now implements the `Keyring<Json>` type ([#194](https://github.com/MetaMask/accounts/pull/194))
  - The class does not extend `EventEmitter` anymore.
  - The `LedgerKeyring.accounts` class variable is now a `readonly Hex[]` array.
  - The `addAccounts` method signature has been changed
    - An `amount` number parameter is now required to specify the number of accounts to add.
    - The method now returns a promise resolving to an array of `Hex` addresses.
  - The `unlock` method now returns `Promise<Hex>`
  - The `getAccounts` method now returns `Promise<Hex[]>`
  - The `signTransaction` method now accepts an `Hex` typed value as the `address` parameter
  - The `signMessage` method now accepts an `Hex` typed value as the `withAccount` parameter
  - The `signPersonalMessage` method now accepts an `Hex` typed value as the `withAccount` parameter
  - The `signTypedData` method now accepts an `Hex` typed value as the `withAccount` parameter
  - The `unlockAccountByAddress` method now accepts an `Hex` typed value as the `address` parameter

### Removed

- **BREAKING:** The `exportAccount` method has been removed

• Fixes: LedgerKeyring class from @metamask/eth-ledger-bridge-keyring should implement the Keyring type #143

[mikesposito]

I'm not sure how to handle this case here. The original code returned already unlocked if:

• there is already a this.hdk defined
• hdPath is not passed to the function (i.e. the consumer calls keyring.unlock()

However, since we want this function to return a value of type Hex we must return a valid address. Based on the lines below the actual publicKey returned from this function comes from the device (payload.publicKey) and it's used to construct an HDKey to be stored in this.hdk. So, the new code assumes that we want to return the address derived from the public key stored in this.hdk.publicKey

[mikesposito]

Marking the PR as draft again because a new @metamask/utils major version is being prepared for release, which will ship breaking changes to the Keyring type