test: Fuzz testing RPC (#2740)

test: Fuzz testing RPC #2422 

Fuzzer to test RPC connections, by sending:

- [x] Invalid methods
- [ ] Correct method with invalid formatted data
- [x] Correct method with validly formatted random data

How to run fuzzing
-------------------

```bach
$ cd chain/jsonrpc
$ cargo install cargo-fuzz  # (if not installed)
$ cargo fuzz run fuzz_target_1
```

**Note** there is some bug in Rust, already fixed but still not included in nightly release, somehow related to MacOS Catalina, so fuzzing is possible only in Linux (definitely) and probably on Windows (haven't check). 

References:
rust-lang/rust#63047 (comment)
servo/core-foundation-rs#357

Test plan
---------

1. test query methods with valid random data and ensure RPC response status is 200 and response contains `error` key instead of `result`.
2. test for valid methods with invalid format of data. Should return 200 and `error` in response.
3. test for invalid methods

Cargo.toml

@@ -28,6 +28,7 @@ members = [
     "chain/network",
     "chain/jsonrpc",
     "chain/jsonrpc/client",
+    "chain/jsonrpc/test-utils",
     "test-utils/testlib",
     "test-utils/loadtester",
     "test-utils/state-viewer",

chain/jsonrpc/client/src/lib.rs

@@ -154,8 +154,8 @@ macro_rules! jsonrpc_client {
     ) => (
         $(#[$struct_attr])*
         pub struct $struct_name {
-            server_addr: String,
-            client: Client,
+            pub server_addr: String,
+            pub client: Client,
         }
 
         impl $struct_name {

chain/jsonrpc/fuzz/.gitignore

@@ -0,0 +1,4 @@
+
+target
+corpus
+artifacts