Add tests & Service's Configuration has optional fields that shouldn't be optional (#4842)

Related to #4776 
Related to https://github.com/paritytech/polkadot/pull/832

To summarize the changes:
1. I did not manage to validate with types the service's Configuration. But I did reduce the possibility of errors by moving all the "fill" functions to their respective structopts
2. I split params.rs to multiple modules: one module params for just CLI parameters and one module commands for CLI subcommands (and RunCmd). Every command and params are in their own file so things are grouped better together and easier to remove
3. I removed the run and run_subcommand helpers as they are not helping much anymore. Running a command is always a set of 3 commands: 1. init 2. update config 3. run. This still allow the user to change the config before arguments get parsed or right after.
4. I added tests for all subcommands.
5. [deleted]

Overall the aim is to improve the situation with the Configuration and the optional parameters, add tests, make the API more consistent and simpler.

substrate/client/cli/src/params/mod.rs

@@ -0,0 +1,65 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+mod import_params;
+mod transaction_pool_params;
+mod shared_params;
+mod node_key_params;
+mod network_configuration_params;
+
+use std::str::FromStr;
+use std::fmt::Debug;
+
+pub use crate::params::import_params::*;
+pub use crate::params::transaction_pool_params::*;
+pub use crate::params::shared_params::*;
+pub use crate::params::node_key_params::*;
+pub use crate::params::network_configuration_params::*;
+
+/// Wrapper type of `String` that holds an unsigned integer of arbitrary size, formatted as a decimal.
+#[derive(Debug, Clone)]
+pub struct BlockNumber(String);
+
+impl FromStr for BlockNumber {
+	type Err = String;
+
+	fn from_str(block_number: &str) -> Result<Self, Self::Err> {
+		if block_number.chars().any(|d| !d.is_digit(10)) {
+			Err(format!(
+				"Invalid block number: {}, expected decimal formatted unsigned integer",
+				block_number,
+			))
+		} else {
+			Ok(Self(block_number.to_owned()))
+		}
+	}
+}
+
+impl BlockNumber {
+	/// Wrapper on top of `std::str::parse<N>` but with `Error` as a `String`
+	///
+	/// See `https://doc.rust-lang.org/std/primitive.str.html#method.parse` for more elaborate
+	/// documentation.
+	pub fn parse<N>(&self) -> Result<N, String>
+	where
+		N: FromStr,
+		N::Err: std::fmt::Debug,
+	{
+		self.0
+			.parse()
+			.map_err(|e| format!("BlockNumber: {} parsing failed because of {:?}", self.0, e))
+	}
+}