Client-side and server-side dispatch cache should report different costs

[jakedt]

The client-side cache should report 0 downstream dispatches, and the server side cache should report 1. Right now it's hard-coded to always report zero:

https://github.com/authzed/spicedb/blob/ac17a3b5b03ccf34d4594e73587072e2c3a5ca1b/internal/dispatch/caching/caching.go#L133

This value should be configurable and the client-side (before remote dispatch) instantiation should use 0 and the server side (after remote dispatch) should use 1.