Support for Rango and (almost) Changelly on Solana

[NickKelly1]

• Almost finish adding support for Changelly on Solana, waiting for response from Changelly team
• Finish adding (untested) support for Rango on Solana
• Added support for third-party-signed Solana swap transactions (required by Rango) and Legacy transactions (also Rango)
• Added a whole lot of extra debug and warning logs for Changelly and Rango
• Fix some issues with the Changelly API
• Add types for all relevant Changelly API endpoints and documentation links
• Add @solana/spl-token as a dependency

Only swap of native SOL to token on other network via Changelly has been tested.