feat: code-generate the system Nexus operations registry

[spkane31]

Related issues

Follow up for #1017. There will be a third required PR to temporalio/api-go to trigger the CI job defined in this repo

What changed?

Replaces the originally hand crafted systemNexusOps map with code generation. New "exposed" operations from go.temporal.io/api will not need manual updates.

Checklist

Tests

• TestParseDiscoversWorkflowService, TestGenerateCode — generator unit tests.
• TestSystemNexusGenUpToDate — fails if the generated file is stale after an api-go bump.

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