ollamarunner: Provide mechanism for backends to report loading progress

This enables the runner to report progress back to the Ollama server,
both for showing status to the user and also to prevent the server
from killing the runner if it thinks things have stalled.

Most of the infrastructure was already there, this extends it to
be available to the backends.

runner/ollamarunner/runner.go

@@ -783,6 +783,9 @@ func Execute(args []string) error {
 	}
 
 	params := ml.BackendParams{
+		Progress: func(progress float32) {
+			server.progress = progress
+		},
 		NumThreads:     *threads,
 		NumGPULayers:   *numGPULayers,
 		MainGPU:        *mainGPU,