docs: add ADR-029 for gRPC TLS and authentication design

Adds a design document (plan of record) for securing the gRPC channel
between the janitor controller and janitor-provider. Covers server-side
TLS via cert-manager and Kubernetes ServiceAccount token authentication,
with options analysis for alternative approaches.

Signed-off-by: JT Schelling <jschelling@nvidia.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: JT Schelling <jschelling@nvidia.com>

Author: jtschelling