cli: prompt for Solana Mainnet RPC override during add-chain

[evgeniko]

clone of #790 due to CI issues with fork PRs

This pull request refactors and improves how CLI prompts and configuration overrides are handled, especially for Solana Mainnet deployments. It introduces reusable prompt utilities, centralizes override logic, and enhances user guidance for setting Solana RPC endpoints.

Prompt utilities and confirmation handling:

• Adds a new prompts.ts module with reusable promptLine and promptYesNo functions for user input, replacing direct usage of readline throughout the CLI. (cli/src/prompts.ts [1] cli/src/index.ts [2] [3]
• Updates confirmation prompts to use the new promptYesNo function, simplifying code and improving consistency. (cli/src/index.ts cli/src/index.tsL4734-R4737)

Overrides management and Solana Mainnet guidance:

• Moves override loading logic to a new overrides.ts module with a loadOverrides function, centralizing configuration file handling. (cli/src/overrides.ts [1] cli/src/index.ts [2] [3]
• Adds a promptSolanaMainnetOverridesIfNeeded function to guide users to set a dedicated Solana Mainnet RPC endpoint in overrides.json, improving deployment reliability. This prompt is triggered automatically when deploying to Solana Mainnet, unless skipped with a CLI flag. (cli/src/overrides.ts [1] cli/src/index.ts [2]