rpc: move client-related code to a separate package

This includes Client struct with RPC methods and
BalanceGetter implementation with NeoSCAN.

pkg/rpc/client/doc.go

@@ -0,0 +1,69 @@
+/*
+Package client implements NEO-specific JSON-RPC 2.0 client.
+This package is currently in alpha and is subject to change.
+
+Client
+
+After creating a client instance with or without a ClientConfig
+you can interact with the NEO blockchain by its exposed methods.
+
+Some of the methods also allow to pass a verbose bool. This will
+return a more pretty printed response from the server instead of
+a raw hex string.
+
+An example:
+  endpoint := "http://seed5.bridgeprotocol.io:10332"
+  opts := client.Options{}
+
+  c, err := client.New(context.TODO(), endpoint, opts)
+  if err != nil {
+	  log.Fatal(err)
+  }
+
+  if err := c.Ping(); err != nil {
+	  log.Fatal(err)
+  }
+
+  resp, err := c.GetAccountState("ATySFJAbLW7QHsZGHScLhxq6EyNBxx3eFP")
+  if err != nil {
+	  log.Fatal(err)
+  }
+  log.Println(resp.Result.ScriptHash)
+  log.Println(resp.Result.Balances)
+
+TODO:
+	Merge structs so can be used by both server and client.
+	Add missing methods to client.
+	Allow client to connect using client cert.
+	More in-depth examples.
+
+Supported methods
+
+	getblock
+	getaccountstate
+	getunspents
+	invokescript
+	invokefunction
+	sendrawtransaction
+	invoke
+	getrawtransaction
+
+Unsupported methods
+
+	validateaddress
+	getblocksysfee
+	getcontractstate
+	getrawmempool
+	getstorage
+	submitblock
+	gettxout
+	getassetstate
+	getpeers
+	getversion
+	getconnectioncount
+	getblockhash
+	getblockcount
+	getbestblockhash
+
+*/
+package client