Add TruffleRuby in CI

[eregon]

On top of #36 to get those fixes.
Still needs a small fix in TruffleRuby to add exception: keyword to StringIO first, cc @andrykonchin (oracle/truffleruby#2247 (comment)).

[eregon]

I should have marked this as a Draft PR since it's not ready yet, sorry.

[eregon]

Should be good now, the next truffleruby-dev build will have the needed fix (oracle/truffleruby@00454d0) and should pass CI.

[eregon]

@sorah Could you review and merge this?

The CI passes: https://github.com/eregon/grpc_kit/runs/7992255982?check_suite_focus=true

[sorah]

We're happy to run a CI against truffleruby but could you add separate job for truffleruby? because at this moment we don't want to (and we can't) guarantee any changes works well on truffleruby, while we fully support MRIs.

[eregon]

It's already its own "CI job" (https://github.com/eregon/grpc_kit/runs/7992255982?check_suite_focus=true), I'm not sure what you mean.
But I can use a different entry under jobs:, yes.

[eregon]

I pushed that.

[sorah]

But I can use a different entry under jobs:, yes.

Yes I meant this

[eregon]

Thanks!