feature: Structured parameters config (near#8384)

This is a part of near#8264.

This PR introduces structured values for Rationals in the parameter config in the form
```yaml
burnt_gas_reward: {
  numerator: 3,
  denominator: 10,
}
```

To support this in the code, we now interpret parameter values as one of
```rust
enum ParameterValue {
    Null,
    U64(u64),
    Rational { numerator: isize, denominator: isize },
    String(String),
}
```

In the future the plan is to extend this to support Fee structure and Weighted parameters.

One open Rust-related question here:
- [ ] What is a good way to deduplicate code between `get_number`, `get_string`, `get_rational` methods?

Things to finish:
- [x] Tests for the Rational type

core/primitives-core/src/parameter.rs

@@ -18,10 +18,8 @@ use crate::config::ActionCosts;
 #[strum(serialize_all = "snake_case")]
 pub enum Parameter {
     // Gas economics config
-    BurntGasRewardNumerator,
-    BurntGasRewardDenominator,
-    PessimisticGasPriceInflationNumerator,
-    PessimisticGasPriceInflationDenominator,
+    BurntGasReward,
+    PessimisticGasPriceInflation,
 
     // Account creation config
     MinAllowedTopLevelAccountLength,

core/primitives/res/runtime_configs/parameters.yaml

@@ -3,10 +3,14 @@
 # The diffs are stored in files named `NN.txt`, where `NN` is the version.
 
 # Gas economics config
-burnt_gas_reward_numerator: 3
-burnt_gas_reward_denominator: 10
-pessimistic_gas_price_inflation_numerator: 103
-pessimistic_gas_price_inflation_denominator: 100
+burnt_gas_reward: {
+  numerator: 3,
+  denominator: 10,
+}
+pessimistic_gas_price_inflation: {
+  numerator: 103,
+  denominator: 100,
+}
 
 # Account creation config
 min_allowed_top_level_account_length: 32

core/primitives/res/runtime_configs/parameters_testnet.yaml

@@ -1,8 +1,12 @@
 # Gas economics config
-burnt_gas_reward_numerator: 3
-burnt_gas_reward_denominator: 10
-pessimistic_gas_price_inflation_numerator: 103
-pessimistic_gas_price_inflation_denominator: 100
+burnt_gas_reward: {
+  numerator: 3,
+  denominator: 10,
+}
+pessimistic_gas_price_inflation: {
+  numerator: 103,
+  denominator: 100,
+}
 
 # Account creation config
 min_allowed_top_level_account_length: 0