feat: dynamic config struct

[tore-statsig]

Adds a struct representing a DynamicConfig. DynamicConfig is typically a class returned by the getConfig and getExperiment methods, which has some accessor methods for metadata and convenience methods for getting values from your config.

For now, ive just included metadata and the value

Bonus - this adds support for groupName, and passes the test for consistency with other server sdks:

 PASS  ../__tests__/common-rulesets-based/dynamic-config-group-name.test.ts
  Dynamic Config Group Name
    Elixir
      ✓ getConfig + Passes Targeting = null (2 ms)
      ✓ getConfig + default = null (1 ms)
      ✓ getExperiment + In Control Group = Control (1 ms)
      ✓ getExperiment + In Control Group = Test (1 ms)
      ✓ getExperiment + In Control Group = Test2
      ✓ getExperiment + Fail Targeting = null (1 ms)
      ✓ getExperiment + Not Allocated = null (1 ms)

[scohen]

there are too many default params here.