feature: add feature flag guard for new account select component

[jackson-dean]

Adds a guard for the new sub account features that makes it "opt in." The reason we do this is because there are breaking changes that external developers will need to do some migration work in order to use it.

We will have a follow up PR in the default frontend (https://node.deso.org), that gives a clear example of how to change the code in order to use the new sub account features. Note that this is only applicable for apps using the legacy iframe API. Anyone using the new desojs lib will simply pick up the new features automatically.

[jackson-dean]

Current dependencies on/for this PR:

• main
  • PR #262
  • PR #263
    • PR #264
    • PR #274
    • PR #266
      • PR #272
      • PR #268 👈
      • PR #269
        • PR #270
        • PR #273
          • PR #277
          • PR #279
        • PR #271
      • PR #267
    • PR #265

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