Decouple node chain-specs from runtime types

[michalkucharczyk]

Removing references to runtime's RuntimeGenesisConfig struct on the node side is not enough to decouple the node/client from the native-runtime entirely.

There are still some runtime-related left-overs required on the node side to build the chain-spec. Examples of the types (or consts) that one needs to know are as follows (example):

• AccountId,
• HostConfiguration,
• SessionKeys (BabeId, GrandpaId etc)
• westend::BABE_GENESIS_EPOCH_CONFIG

This applies to every complex type defined in runtime.

These types need to be known at time of creating the json chain-spec. One needs to know what the types are to properly put them into the json format of runtime genesis config. Assuming that we don't want to craft json files manually, this requires building the helper tool for each runtime (as it was done for fellowship runtimes here) or incorporating the runtime deps into client code.

To overcome this problem, I propose to move the creation of the JSON representation of the runtime genesis config into the runtime itself, and exposing multiple configurations through the GenesisBuilder API.
GenesisBuilder implementation may encapsulate any number of predefined named configurations (e.g. "dev", "testnet", "staging", "alice"), and make them accessible via the runtime API.
The named configuration within runtime would be provided in the form of JSON patch (meaning that only the customized fields would be included into it). Refer to sc_chain_spec doc to find out more about genesis config patch format

The GenesisBuilder API could be updated as follows (draft):

fn get_preset_names() -> Vec<String>;
fn get_preset(name: Option<String>) -> serde_json::Value;
fn build_state(value: serde_json::Value);

The ChainsSpecBuilder (introduced in #1256) would be extended with one additional method: with_named_genesis_config_patch, allowing chain-spec construction with the following code:

type RococoChainSpec = ChainSpecBuilder<(), Extensions>;

RococoChainSpec::builder(rococo::WASM_BINARY, Extensions::default()) 
   .with_name("Rococo Staging Testnet")
   .with_id("rococo_staging_testnet")
   .with_chain_type(ChainType::Live)
   .with_named_genesis_config_patch("staging")
   .with_telemetry_endpoints(...)
   .with_boot_nodes(...)
   .with_protocol_id(DEFAULT_PROTOCOL_ID)
   .build()

The above sequence interacts with the GenesisBuilder API of provided runtime, and builds the chain-spec.

(cc: @kianenigma)

Originally posted by @michalkucharczyk in #62 (comment)

part of: #25

[michalkucharczyk]

This will require to add Keyring support into runtime:

polkadot-sdk/substrate/primitives/keyring/src/sr25519.rs

Lines 31 to 40 in c284a93

	pub enum Keyring {
	Alice,
	Bob,
	Charlie,
	Dave,
	Eve,
	Ferdie,
	One,
	Two,
	}

Consequently, Pair::from_string will be required:

polkadot-sdk/substrate/primitives/core/src/crypto.rs

Line 992 in c284a93

fn from_string(s: &str, password_override: Option<&str>) -> Result<Self, SecretStringError> {

Randomness is not needed as the DEV_PHRASE is hard-coded.