Revisit public methods

[ifropc]

We should revisit methods we export publicly. Some methods available by the rpc-client should not be there, and instead should be moved to the cli. The main reason behind it is we want an rpc-client to be a "thin" client with no extra functionality.
Some context:

The lib started by extracting logic from the cli, but we extracted a lot more than just the client and the boundary between two had a lot of cli logic on the rpc client side. We've moved a lot over, but there's still things there.

From the discussion of
stellar/stellar-cli#1494

[willemneal]

I still think we should have a second higher level crate that the CLI uses. It would be useful for anyone who wanted to use this crate separately from the CLI.