cobra/commands: cancel command context on signal

See https://github.com/docker/cli/pull/4599 and
https://github.com/docker/cli/pull/4769.

Since we switched to using the cobra command context instead of
`appcontext`, we need to set up the signal handling that was being
provided by `appcontext`, as well as configuring the context with
the OTEL tracing utilities also used by `appcontext`.

This commit introduces `cobrautil.ConfigureContext` which implements
the pre-existing signal handling logic from `appcontext` and cancels
the command's context when a signal is received, as well as doing
the relevant OTEL config.

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

vendor/github.com/moby/buildkit/util/tracing/env/traceenv.go

@@ -14,10 +14,10 @@ const (
 )
 
 func init() {
-	appcontext.Register(initContext)
+	appcontext.Register(InitContext)
 }
 
-func initContext(ctx context.Context) context.Context {
+func InitContext(ctx context.Context) context.Context {
 	// open-telemetry/opentelemetry-specification#740
 	parent := os.Getenv("TRACEPARENT")
 	state := os.Getenv("TRACESTATE")