Add rustfmt.toml to explicitly set the root edition to 2018

[jeckersb]

Some IDEs/plugins do not proprely infer the edition from the workspace crates, and instead only check the root and default to 2015 since the root Cargo.toml does not have a project in which to declare an edition.

See: rust-lang/rustfmt#4454 (comment)

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jeckersb
Once this PR has been reviewed and has the lgtm label, please assign lalatendumohanty for approval by writing /assign @lalatendumohanty in a comment. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[jeckersb]

Actually hold this for now, this only seems to half-fix the issue for me. Needs more investigation.

[jeckersb]

Nevermind, turns out I had two copies of rustfmt running in parallel; one with edition and one without edition and they were racing each other. I fixed that and now it's able to figure out the edition automatically. Sorry for the noise.