feat: add integration plugin

[bgiori]

Summary

• Add ExperimentPlugin modeled after analytics plugin interface.
• Add addPlugin method to ExperimentClient.
• New plugin, IntegrationPlugin handles getting user identity and tracking exposures
  • initializeWithAmplitudeAnalytics now installs the AmplitudeIntegrationPlugin instead of ConnectorUserProvider and ConnectorExposureTrackingProvider.
• IntegrationManager handles managing IntegrationPlugins for the client.
  • Adds event persistence to experiment SDK
  • Re-implements experiment deduping using session storage rather than in-memory storage.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: NO

[zhukaihan]

Can plugins make modifications to config, or do we want to pass in a copy of config?

[bgiori]

Hmmm, interesting thought. I'm actually not quite sure what the right answer is. I kind of like the flexibility that they could change config.