feat(host): Execute kona-client in-process

[clabby]

Overview

Currently, the kona-host binary shells out to invoke the kona-client binary, rather than executing it in-process. This was done towards the beginning in order to test the kona-preimage API natively in a similar way that the FPVMs use, but there are a few problems:

• The current BasicKernelInterface::write & BasicKernelInterface::read interfaces take an owned FileDescriptor, which is inflexible for a few reasons:
  • It contains mappings that are invalid on the native architecture or non fault proof VMs (i.e. zkVMs). This is a poor abstraction for re-use.
  • On the NativeIO implementation, we have to take a FileDescriptor type, which is bad for reasons described in feat(common): Improve native IO API #553.
• The kona-host implementation therefore has to keep the pipe files for the client program alive, in order to guarantee that the pipe remains open for the duration of the asynchronous child process' lifetime.

Instead of doing this, we should change kona-host up so that kona-client runs natively within the process. This will remove the need for the NativeIO entirely in kona-common, and then we can split kona-common into client program utilities and FPVM-specific logic. This should remove the need to solve #553.

[samlaf]

@clabby few questions here as I'm trying to understand how kona works:

1. First am I thinking correctly about kona that there are 2 ways to run kona-host: as part of FPVM, or natively on hardware (which has goal to produce an rust implementation of op-node)?
2. Then the goal of this issue is to make the second native hardware mode (is that what the "Online" providers are for?) faster?