feat: add interactive startup wizard

[darwvin-dev]

Summary:
Start an interactive startup wizard when SIPp is launched without arguments on an interactive terminal. The wizard helps users build common -sn and -sf command lines and preserves the existing help behavior for non-interactive no-argument runs.

Reasoning:
Launching SIPp without arguments is a common first-touch path, especially for users who are exploring the built-in scenarios or validating a new build. Today that path drops straight to the generic help output, which is complete but still leaves the user to translate their intent into a working command line.

This wizard is meant to reduce that startup friction without changing batch or scripted behavior. It only runs when both stdin and stdout are interactive, it keeps the current help path for non-interactive usage, and it only offers scenario and transport choices that are supported by the current build.

What is included:

• Interactive prompts for common embedded and file-based scenario starts.
• Transport, remote host, request URI, call rate, and basic limit prompts.
• A final rendered command preview before execution.
• Existing no-argument help behavior retained for non-interactive runs.

What is not included:

• No Call-ID generation changes. Those are split into a separate PR for review on their own merits.

[lemenkov]

I like the idea tbh. I remember myself trying it for the first time.