refactor(prover-alloy): extract common client builder logic

[aso20455]

Extracted duplicated HTTP client creation logic into a reusable helper function create_client_builder. This eliminates code duplication between build_alloy_fill_provider and AlloyProvider::new, improving maintainability and reducing the risk of inconsistencies when making changes to the client configuration

[atanmarko]

Hi @aso20455, thank you for your contribution. Unfortunately, we are still waiting for the approval from our legal department before we can merge external contributors PRs.