plugin manager: options for sample rate, audio buffer size, and MIDI buffer size

[atsushieno]

We will have to implement audio graph rebuilder per configuration, and these options will be useful for dogfooding.

[atsushieno]

Related: https://github.com/atsushieno/aap-core/blob/main/docs/design/PLUGIN_MANAGER_APP.md#configuration-changes

[atsushieno]

Maybe this should be worked on after #163 is done.