refactor: type Transaction to include all tx
BREAKING CHANGE: `Transaction` type has been redefined to include all
transactions and `SubmittableTransaction` was created to define the old
value. The following functions which only handle transactions to be
submitted now use `SubmittableTransaction`:
* `Client.autofill`
* `Client.submit`
* `Client.submitAndWait`
* `Client.prepareTransaction`
* `getSignedTx`
* `isAccountDelete`