Implement SDK polling, in-memory cache

Fixes: #issue

Motivation and Context

The client SDK needs to know when an experiment's configuration changes. For example, if the experiment percent exposure changes from 100% to 0%, the SDK assign function will stop assigning variations and always return null.

Description

This PR implements polling of the RAC endpoint to retrieve the most up-to-date configurations (the design doc also considered a push-based approach instead of polling). Once the RAC is retrieved, the SDK stores it in a local in-memory cache:

"experiment1": => { ...config }
"experiment2": => { ...config }

How has this been tested?

When invoking the assign function, it uses the configurations from the local cache (except on the first request because the poller hasn't started yet)
```
Getting assignment
Getting configurations from network request
Got assignment variation-1
```

Getting assignment Getting configurations from local cache Got assignment variation-1

Polling experiment configurations... Getting assignment Getting configurations from local cache Got assignment variation-1


- Verified query params are sent in the HTTP request:
  - `apiKey`: "test-api-key"
  - `sdkName`: "node-server-sdk"
  - `sdkVersion`: "1.0.0"

- The `assign function responds to RAC changes on the backend

Getting assignment Got assignment variation-1

------ CHANGE to ZERO PERCENT EXPOSURE ----

Getting assignment Got assignment null



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Eppo-exp / node-server-sdk