Improve IBC Callbacks Helpers

[chipshort]

You could create an interesting API if you allow something like transfer_msg.with_src_callback(env, gas_limit) instead of writing it to the memo directly here. The method .with_src_callback would also return an error if the memo is not empty to ensure we are not overriding important info.

Provide something like a TransferMsgBuilder that takes everything except the memo and provides with_src_callback, with_dst_callback, with_callbacks, with_memo and without_memo to convert to a TransferMsg. That way you cannot misuse it accidentally.

Originally posted by @chipshort in #2025 (comment)