This allows to:
1. Have a preset logger that is already correctly matching the environment (console vs Unity).
2. Removes the need for explicit `SpacetimeDBClient.CreateInstance(...)` which is particularly awkward to use and easy to forget with singleton as it doesn't return a result as a factory method name could suggest. Instead, `SpacetimeDBClient.instance` is available on first use.
3. Slightly simplifies dependencies between classes, e.g. `ClientCache` doesn't need a circular dependency on `SpacetimeDBClient`, making future maintenance and changes a bit easier.
Ingvar Stepanyan