RPC Differences Documentation and RPC Testing Toolkit (#241)

* Added RPC testing toolkit with docs

* Add documentation

* fix comments

---------

Co-authored-by: Amar Singh <asinghchrony@protonmail.com>

docs/modules/ROOT/nav.adoc

@@ -4,6 +4,7 @@
 * EVM Template Guides
 ** xref:guides/contract_migration.adoc[Contract Migration]
 ** xref:guides/predeployed_contracts.adoc[Predeployed Contracts]
+** xref:guides/rpc_differences.adoc[RPC Differences]
 * Runtimes
 ** xref:runtimes/generic.adoc[Generic Runtime]
 * Runtime Descriptions

docs/modules/ROOT/pages/guides/rpc_differences.adoc

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+:source-highlighter: highlight.js
+:highlightjs-languages: rust
+:github-icon: pass:[<svg class="icon"><use href="#github-icon"/></svg>]
+
+= RPC Differences
+
+The EVM in a Substrate node has almost the same RPC calls common to all nodes. But some of them are not applicable for Frontier.
+
+These are the calls that behave differently than would be expected by regular Ethereum nodes:
+
+* `eth_sign` / `eth_signTransaction` -- these methods do not exist in the Frontier and this template because it, unlike Ethereum nodes, does not hold any keys.
+* `eth_syncing` -- it can have additional fields `warp_chunks_amount` and `warp_chunks_processed` that describe warp syncing process.
+
+Other calls comply to the same scheme as Ethereum nodes provide.

evm-template/rpc-testing/README.md

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+## RPC Testing Toolkit
+
+This is a Postman collection to test your the RPC deployment of you smart contract. To use it you should prepare your environment and test toolkit for testing:
+
+* Open the collection in Postman, and open the variables in it.
+* Fill the `url` with HTTP RPC url and `chain_id` with your chain's chain id.
+* Obtain three funded accounts. You may provide them through the chainspec. 
+  * Put first address to the `alice_addr` variable and their balance to the `alice_balance`.
+  * Make a transfer transaction from the second address to any address.
+    * Put its hash to the `tx_hash` variable
+    * Put the block number of this transaction to `tx_block_number` and `block_number`. Put the number of transactions in this block (highly likely it will be 1) to the `block_tx_count`.
+    * Put the hash of the block with this transaction to `tx_block_hash` and `block_hash`.
+    * Put the index of the transaction in the block to `tx_index`. Highly likely it will be 0.
+    * Put the address you have sent the transaction from to `tx_from` variable.
+  * Take a third address and deploy the smart contract `TestContract.sol` from it.
+    * Put the address of the deployed contract to the `test_contract` variable.
+    * Put the code from the compiled contract to the `contract_code` variable. It can be found in a resulting JSON file under `deployedBytecode` key.
+
+When you complete these preparation steps, run the requests from the collection. If some of them are not passing, run them as a single request and see what the request have returned to debug it.

evm-template/rpc-testing/TestContract.sol

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+contract Storage {
+    uint pos0;
+
+    constructor() {
+        pos0 = 1234;
+    }
+
+    // You can read from a state variable without sending a transaction.
+    function get() public view returns (uint) {
+        return pos0;
+    }
+}
+