feat: export ProtoDescriptorHelper (#245)

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##### Checklist
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[x]. -->

- [ ] `npm test` passes
- [ ] tests and/or benchmarks are included
- [ ] documentation is changed or added
- [ ] commit message follows commit guidelines

##### Affected core subsystem(s)
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##### Description of change
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced module interface by adding exports from the
`ProtoDescriptorHelper`, providing additional functionality.

- **Refactor**
- Updated the inheritance structure of the
`ChildStaticMultiInstanceProto` class to improve its relationship within
the class hierarchy.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

core/core-decorator/test/fixtures/decators/ChildService.ts

@@ -11,7 +11,7 @@ export class ChildSingletonProto extends ParentSingletonProto {}
 })
 export class ParentStaticMultiInstanceProto {}
 
-export class ChildStaticMultiInstanceProto extends ParentSingletonProto {}
+export class ChildStaticMultiInstanceProto extends ParentStaticMultiInstanceProto {}
 
 
 @MultiInstanceProto({

core/metadata/index.ts

@@ -17,6 +17,7 @@ export * from './src/model/graph/ProtoSelector';
 export * from './src/model/ProtoDescriptor/AbstractProtoDescriptor';
 export * from './src/model/ProtoDescriptor/ClassProtoDescriptor';
 export * from './src/model/ModuleDescriptor';
+export * from './src/model/ProtoDescriptorHelper';
 
 import './src/impl/ModuleLoadUnit';
 import './src/impl/EggPrototypeBuilder';