chore: enable setting a static gas price via NodeConfig

[grandima]

Closes #1581

Release notes

In this release, we:

• Added a possibility to configure --gas-price-change-percent via NodeConfig.

Checklist

• [x] All changes are covered by tests (or not applicable)
• [x] All changes are documented (or not applicable)
• [x] I reviewed the entire PR myself (preferably, on GH UI)
• [x] I described all Breaking Changes (or there's none)

---

[!NOTE] Introduce gas_price_change_percent in NodeConfig, pass it via --gas-price-change-percent, and add an e2e test verifying static gas price across blocks.

• Test Helpers:
  • Add NodeConfig.gas_price_change_percent (default 0).
  • Pass through to node via --gas-price-change-percent in fuel_bin_service::ExtendedConfig::args_vec.
• E2E Tests:
  • Update e2e/tests/providers.rs to configure static gas price (starting_gas_price + gas_price_change_percent: 0).
  • Verify estimate_gas_price remains constant after producing blocks.

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[grandima]

@hal3e please review