feat: expose custom plugin setup for editing

[sagarkt]

Summary

• add an opt-in fetchPluginSteps field to the v2 plugin detail request
• return persisted steps, variables, conditions, scripts, and path/port mappings for detailed SHARED plugin versions
• keep preset internals and ordinary plugin discovery responses unchanged
• update the previous-version latest flag in the version-creation transaction
• document the edit payload and cover configuration assembly with unit tests

Testing

• GOCACHE=/private/tmp/devtron-43-go-cache go test ./pkg/plugin/... ./api/restHandler ./api/router
• GOCACHE=/private/tmp/devtron-43-go-cache go vet ./pkg/plugin/... ./api/restHandler ./api/router
• parsed specs/plugin/global.yaml and verified all component schema references

Related to #6916

[bito-code-review]

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