Easy WASI Command Runner

[G4Vi]

To continue expanding Extism from a plugin system to a framework, we could add a WASI command module runner. A WASI command module runner would allow using Extism to run existing programs in the wild in addition to plugins built with the various PDKs.

The interface would take in a buffer to use as STDIN data and return two buffers out, STDOUT and STDERR. Setting this up in all WASM runtimes is non-trivial. Such interface would be convenient for creating a library function around a command module for example.

Prototype implementation: https://github.com/extism/extism/tree/g4vi/wasi-command-runner