Invoke custom commands from custom proc/txn

[yrajas]

Adds ability to invoke custom raw string commands and object commands from within custom procedures and transactions.

• New APIs in CustomFunctions to invoke custom commands from custom procedures and transactions
• CustomProcedure registration updated to take in a delegate similar to custom transaction registration
• RespServerSession reference added to CustomFunctions to enable parsing of custom commands and invoking underlying API
• Sample custom procedure and transaction that invoke custom raw string and object commands
• Corresponding tests.