Assignment hooks and asynchronous method to enable sticky assignments

[petzel]

Motivation and Context

Customers have requested the ability to ensure a customer continues to receive a treat once assigned, even if the feature flag/experiment has concluded and is no longer assigning any users.

Description

This PR creates hooks which are invoked prior to and after assignment. If the pre assignment hook returns a string, it will be used/returned as the variation. The post assignment hook is a way to handle saving/persistence of assigned variations for a given user.

Example of how this can be used:

const key = `${subject}-my-cool-experiment`;
const variation = await eppoClient.getAssignmentWithHooks(
  'user-123',
  'my-cool-experiment',
  {},
  {
    onPreAssignment(subject: string): Promise<string> {
      const overriddenTreatment = database.get(key);
      if (overriddenTreatment != null && overriddenTreatment !== '') {
        return Promise.resolve(overriddenTreatment);
      }
      return null;
    },

    onPostAssignment(variation: string): Promise<void> {
      database.save(key, variation);
    },
  },
);