Offchain Workers: Example Pallet (#4989)

* Example of offchain worker pallet.

* Fix compilation issues.

* Use serde_json to parse JSON.

* Add some basic tests.

* Working on docs.

* Fix compilation

* Finish docs for signed.

* Work on unsigned send.

* Add some tests and missing docs.

* Add example of StorageValueRef

* Add weight.

* Extra \n

* Fix im-online test.

* Bump runtime.

* Fix tests.

* Apply suggestions from code review

Co-Authored-By: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>
Co-Authored-By: Gavin Wood <gavin@parity.io>

* Address review comments.

Co-authored-by: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>
Co-authored-by: Gavin Wood <github@gavwood.com>

substrate/Cargo.lock

@@ -3722,6 +3722,12 @@ version = "0.1.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "72ef4a56884ca558e5ddb05a1d1e7e1bfd9a68d9ed024c21704cc98872dae1bb"
 
+[[package]]
+name = "nohash-hasher"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "721a2bf1c26159ebf17e0a980bc4ce61f4b2fec5ec3b42d42fddd7a84a9e538f"
+
 [[package]]
 name = "nohash-hasher"
 version = "0.2.0"
@@ -4149,6 +4155,21 @@ dependencies = [
  "sp-std",
 ]
 
+[[package]]
+name = "pallet-example-offchain-worker"
+version = "2.0.0"
+dependencies = [
+ "frame-support",
+ "frame-system",
+ "parity-scale-codec",
+ "serde",
+ "serde_json",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std",
+]
+
 [[package]]
 name = "pallet-finality-tracker"
 version = "2.0.0"
@@ -6215,7 +6236,7 @@ dependencies = [
  "linked_hash_set",
  "log 0.4.8",
  "lru 0.4.3",
- "nohash-hasher",
+ "nohash-hasher 0.2.0",
  "parity-scale-codec",
  "parking_lot 0.10.0",
  "prost",
@@ -9168,14 +9189,14 @@ checksum = "d089681aa106a86fade1b0128fb5daf07d5867a509ab036d99988dec80429a57"
 
 [[package]]
 name = "yamux"
-version = "0.4.3"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d73295bc9d9acf89dd9336b3b5f5b57731ee72b587857dd4312721a0196b48e5"
+checksum = "902f4cee32c401c211b6b69f4a3f6f4cf3515644db5bd822cf685a7dbd6201f9"
 dependencies = [
  "bytes 0.5.4",
  "futures 0.3.4",
  "log 0.4.8",
- "nohash-hasher",
+ "nohash-hasher 0.1.3",
  "parking_lot 0.10.0",
  "rand 0.7.3",
  "thiserror",