preflightlib : add support for fee calculations

[tsachiherman]

What ?

The rs-soroban-env now contains library functions to calculate the fees needed by a transaction, based on it's resources.
Our goal is to integrate that into simulateTransaction endpoint in soroban-rpc. This issue is about the first step in the process:
We need to expose the newly added function via the preflightlib so that soroban-rpc would be able to call it.
This issue is focused around adding support for calling the new method, plus ensuring that soroban-rpc is capable of importing and calling. It doesn't cover any plumbing around the simulateTransaction endpoint itself ( that would be handled in a separate ticket ).

Epic

#519