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

• All changes are covered by tests (or not applicable)
• All changes are documented (or not applicable)
• I reviewed the entire PR myself (preferably, on GH UI)
• 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.

^{Written by Cursor Bugbot for commit 23c6476. This will update automatically on new commits. Configure here.}

[grandima]

@hal3e please review