add string buffer for stdout and stderr

[jenniferwills]

This PR adds support for using a string with a byte limit as the stdout and stderr for WasiCtx and WasiCtxBuilder.

Usage:

stdout_str = ""
stderr_str = ""
WasiCtxBuilder.new
          .inherit_stdin()
          .set_stdout_buffer(stdout_str, 50000)
          .set_stderr_buffer(stderr_str, 50000)
          .build

[saulecabrera]

Thanks for putting this together! I left a couple of suggestions and comments before landing this.

[saulecabrera]

Thanks for iterating on this, this looks good to me. I'm not sure what's going on with the 3.0 runners though, I don't think it's related to this PR. I'll take a look.

[saulecabrera]

Note on the runners: 3.0 is no longer part of the stable-ruby-versions that are fetched by oxidize-rb/actions.

From: https://github.com/oxidize-rb/actions/blob/main/fetch-ci-data/evaluate.rb#L54

["3.1","3.2","3.3","head"]