feat: Support registering external contracts

This starts to add support for registering external smart contracts
with CDP.

This will enable developers to use these register smart contracts
to invoke, read, and create webhooks associated with the contract!

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 * Add support for fetching address reputation.
+* Add support for registering, updating, and listing smart contracts that are
+  deployed external to CDP.
 
 ## [0.12.0] - Skipped
 

lib/coinbase/errors.rb

@@ -84,6 +84,14 @@ def initialize(msg = 'Transaction must be signed')
     end
   end
 
+  # An error raised when attempting to manage an external contract on-chain,
+  # e.g. sign the deployment tx, deploy the contract, etc...
+  class ManageExternalContractError < StandardError
+    def initialize(action = 'manage')
+      super("Cannot #{action} external smart contract")
+    end
+  end
+
   # An error raised when an address attempts to sign a transaction without a private key.
   class AddressCannotSignError < StandardError
     def initialize(msg = 'Address cannot sign transaction without private key loaded')