[feature] expose ModelManagerConfiguration

[Giwayume]

Is your feature request related to a problem? Please describe.
Export ModelManagerConfiguration interface from ModelClient.ts to root import so end user may use it.

Describe the solution you'd like
Can't do this without hax:

import { ModelManagerConfiguration } from '@adobe/aem-spa-page-model-manager';
const config: ModelManagerConfiguration = {};
config.modelClient = whatever;
ModelManager.initialize(config);

Describe alternatives you've considered
I can workaround by using import { ModelManagerConfiguration } from '@adobe/aem-spa-page-model-manager/dist/ModelManager';, but importing anything directly from a dist folder is usually risky behavior as it is more prone to implementation change without notice or documentation.