feat: support `--rpc-url` for local/standalone network testing

[wagmiiii]

Description

Currently, cargo-budget-report relies heavily on public networks (testnet or futurenet). For teams running a local standalone Soroban RPC node (via Docker) to avoid network rate limits or to test custom network fee settings, the tool currently blocks them because the RPC URL cannot be overridden manually.

What "done" looks like

• Add a --rpc-url <URL> argument to the cargo-budget-report CLI.
• Add a --network-passphrase <PASSPHRASE> argument (required if --rpc-url is used).
• If these flags are provided, bypass the testnet/futurenet defaults and simulate transactions against the provided local RPC endpoint.
• Update the crate README.md with documentation for these flags.

Implementation guidelines

• Create a branch: git checkout -b feat/cli-custom-rpc-url
• Update the clap parser in cargo-budget-report/src/main.rs to accept the two new arguments.
• Pipe the custom URL and passphrase into the simulation configuration (whether that's via the native HTTP client or subprocess, depending on the current merged state).
• Add a unit test verifying that the CLI correctly parses the arguments and overrides the network selection.

PR guidelines

• Get assigned before starting.
• PR description must include: Closes #[this issue].

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📋 Before you start

Please read our Code Quality Standards in CONTRIBUTING.md. Before submitting a PR, ensure you run:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

[Cjay-Cyber-2]

@Cjay-Cyber-2 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i would like to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Cjay-Cyber-2 to this issue.

[drips-wave]

Congratulations, @Cjay-Cyber-2! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Cjay-Cyber-2: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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