Update gRPC interface with new methods

[cpacia]

• [x] Network is modified to also return the blockchain height and best hash

• [x] CreateWallet is modified to take a mnemonic string instead of a byte slice seed.

• [x] A CreateTransaction method is added which returns a serialized transaction. The FundTransaction method which exists expects the client to manually build and serialize the transaction. It's much easier on the client to just let the server do this.

• [x] A SweepAccount method is added to sweep all inputs into an address.

• [x] A CurrentAddress method is added to return the last unused external address.

• [x] The TransactionDetails object is modified to include output addresses.

• [x] VerifyAddress and GenerateMnemonicSeed helper methods are added so they don't need to be implemented client side.

[cpacia]

Going to merge. For tests we'll need to build out a mock wallet structure. We can create a separate task for that.