[FEATURE] Graceful defaulting to provided code default

[toddbaert]

Requirements

Previously, we worked on #1646, which was a step in the right direction. In summary, we changed the minimum flag definition to allow for the omission of the defaultVariant:

    "simple-string": {
      "state": "ENABLED",
      "variants": {
        "greeting": "Hello, World!",
        "farewell": "Goodbye, World!"
      },
      "defaultVariant": null // this can now be set to null, or omitted completely instead of a variant; indicating that the code default should be used.
    },

However, this was implemented in a stop-gap manner. In order to facilitate the code-defaulting behavior as simply as possible, our in-process providers and flagd's remote evaluation protocols behave as if such flags do not exist (an error, which triggers the code default). This is not ideal since it pollutes telemetry with errors which are not actually representative of unexpected behavior, and hurts performance by causing stack traces to be produced internally in many cases. Moreover it simply confuses developers when they use detailed resolution APIs.

We need to improve this. This will involve:

• changes to the evaluation.v1 to support omitting a variant/value or otherwise communicating that the code default should be used
  • may require an evaluation.v2 if impossible or impractical to accomplish in a non-breaking way
    • if we are doing an evaluation.v2, we should also consider removing ResolveAll since this is only used by flagd-web, which is being deprecated
• similar changes to flagd's OFREP handler as above (OFREP already supports this, see https://github.com/open-feature/protocol/blob/main/service/adrs/0006-optional-value-for-code-defaults.md and https://github.com/open-feature/protocol/pull/49/changes
• changes to every in-process evaluator
• updates to the test harness here (we will probably need to have a step/annotation for this to progressively migrate it)
• updates to documentation (specifically see this section, and the note contained therein)