Add config file to enable Buildomat CI for illumos (#6768)

## Motivation

As described in #6763, Tokio compiles for the [illumos] operating
system, but we don't presently have automated tests on illumos. We would
like to add illumos CI jobs for Tokio using [Buildomat], a CI system
which supports illumos. Buildomat CI jobs for Tokio will run on
infrastructure contributed by Oxide Computer Company.

In order for Buildomat to watch for commits to the repo, we must first
add a configuration file in `.github/buildomat/config.toml` with the
`enable = true` key. This config file must be present on the repo's main
branch for Buildomat to enable builds for the repo. See [here] for
details.

## Solution

This branch adds a `.github/buildomat` directory containing a config
file and a README summarizing what the configs in that directory are
for, as well as documenting how to get help diagnosing illumos CI
failures.


This branch does *not* add scripts for actually running CI jobs on
Buildomat. Since the config file must be present on the repo's main
branch before Buildomat runs CI jobs for the repo, I'd like to merge the
config file separately from the actual build scripts. This way, I can
actually have the build jobs run on the PR that adds them, making it
easier to ensure everything is working correctly before merging.

Closes #6766, which is obsoleted by this branch.

[illumos]: https://www.illumos.org/
[Buildomat]: https://github.com/oxidecomputer/
[here]:
    https://github.com/oxidecomputer/buildomat/blob/main/README.md#per-repository-configuration

.github/buildomat/README.md

@@ -0,0 +1,20 @@
+# Buildomat illumos CI
+
+This directory contains CI configurations for the [illumos] operating system.
+Tokio's illumos CI jobs are run using [Buildomat], a CI system developed by
+Oxide Computer, which supports illumos. See [the Buildomat README] for more
+details.
+
+## illumos-Specific CI Failures
+
+If your pull request's CI build fails on illumos, and you aren't able to easily
+reproduce the failure on other operating systems, don't worry! The
+[tokio-rs/illumos] team is responsible for maintaining Tokio's illumos support,
+and can be called on to assist contributors with illumos-specific issues. Please
+feel free to tag @tokio-rs/illumos to ask for help resolving build failures on
+illumos 
+
+[illumos]: https://www.illumos.org/
+[Buildomat]: https://github.com/oxidecomputer/buildomat
+[the Buildomat README]: https://github.com/oxidecomputer/buildomat
+[tokio-rs/illumos]: https://github.com/orgs/tokio-rs/teams/illumos

.github/buildomat/config.toml

@@ -0,0 +1,8 @@
+# Repository-level Buildomat configuration.
+# See: https://github.com/oxidecomputer/buildomat#per-repository-configuration
+
+# Enable buildomat. This one should be self-explanatory.
+enable = true
+# Allow CI runs for PRs from users outside the `tokio-rs` organization. Our
+# buildomat jobs don't touch any secrets/keys, so this should be fine.
+org_only = false