feat: anonymous usage telemetry via inline oss-telemetry

Adds a yaegi-safe inline telemetry helper that fires a single
fire-and-forget ping at plugin load. Helps track adoption and version
spread. No persistent identifiers are collected.

Implementation notes:
- inline (no external dep) so Traefik plugin loader does not need to
  resolve a new vendored module
- stdlib-only, no generics, no range-over-int — verified to load under
  yaegi 0.16.x (full plugin import + CreateConfig/New symbol lookup OK)
- avoids `switch{case A,B,C:}` blocks where some yaegi releases
  mis-evaluate comma-separated case lists
- sync.Once guards against amplified pings on Traefik dynamic config
  reloads (which re-instantiate the middleware)

Opt out via any of:
  DO_NOT_TRACK=1
  OSS_TELEMETRY_DISABLED=1
  TRAEFIKOIDC_DISABLE_TELEMETRY=1

main.go

@@ -89,6 +89,7 @@ var defaultExcludedURLs = map[string]struct{}{
 //   - The configured TraefikOidc handler ready to process requests.
 //   - An error if essential configuration is missing or invalid (e.g., short encryption key).
 func New(ctx context.Context, next http.Handler, config *Config, name string) (http.Handler, error) {
+	sendTelemetry(pluginVersion)
 	return NewWithContext(ctx, config, next, name)
 }