walletkit: add SubmitPackage

[bhandras]

This is experimental and is pending on lightningnetwork/lnd#9784

Pull Request Checklist

• PR is opened against correct version branch.
• Version compatibility matrix in the README and minimal required version
  in lnd_services.go are updated.
• Update macaroon_recipes.go if your PR adds a new method that is called
  differently than the RPC method it invokes.

[guggero]

BTW, just as an info: Each sub client of the lndclient now has a method RawClientWithMacAuth which allows you to call any underlying RPC method without needing to add it to the code here first. Could be useful during development.