Set up ability to generate union types for RPC

[THEjacob1000]

Internal change only

AJV Docs: https://ajv.js.org/json-type-definition.html#union-keyword

This PR adds the ability to generate union types without them having to be discriminated unions. Allowing us to define unions like this:

      OrderingConfig: {
        metadata: {
          union: [
            { ref: "KountaConfig" },
            { ref: "PublicApiConfig" },
            { ref: "ZonalConfig" },
          ],
        },
      },

And have it generate out like this:

export type OrderingConfig = KountaConfig | PublicApiConfig | ZonalConfig;

Submitter Checklist

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• Self-reviewed code, removed extraneous comments, debug logging, checked code style
• No change to users after merge (off-by-default configuration, feature flag, alt URL, etc.) and can be disabled if issues arise (if this is not the case we may need stakeholder signoff)
• Changes are documented (README, wiki, etc)
• Automated tests added or existing tests cover the new functionality or changes
• Manually tested changes
• Metrics added to track the usage/performance
• I am confident I can revert this change
• Database migrations are reversible without data loss (if applicable)
• Performance impact is acceptable (if applicable)
• Any new dependencies are justified or have been approved (if applicable)
• This is NOT a high risk change (if it is, please follow the high-risk change process)

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Reviewer Checklist

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• I am ok with code style and functionality
• I have personally tested the feature
• My review was not rushed due to time constraints