Expose Smithy-based Java SDK code generation pipeline similar to AWS SDK JS v3

### Describe the feature

We noticed a significant difference between the AWS SDK JS v3 and AWS SDK Java v2 code generation ecosystems.

In AWS SDK JS v3, the repository publicly exposes a clean Smithy based code generation pipeline via modules like:

* `codegen/sdk-codegen`
* `smithy-aws-typescript-codegen`

This makes it possible to:

* generate SDKs from Smithy AST JSON models
* create custom projections/plugins
* extend service models
* experiment with custom auth schemes and service extensions

However, in AWS SDK Java v2, there does not appear to be an equivalent publicly exposed Smithy Java client codegen plugin or standalone SDK generation pipeline.

While internal Java codegen clearly exists, there seems to be no public equivalent to:

* `typescript-codegen`
* `smithy-aws-typescript-codegen`
* `sdk-codegen`

This creates challenges for teams extending AWS-compatible services because:

* we can generate Smithy AST JSON models successfully
* but cannot generate Java SDK clients using the same modern Smithy projection/plugin workflow available in JS SDK v3
* there is no documented/public `java-client-codegen` plugin equivalent

Request:
Would AWS consider publishing or documenting the Java SDK v2 Smithy-based code generation pipeline similarly to JS SDK v3?

### Use Case

We are extending AWS SDK for Java v2 and noticed that AWS SDK JS v3 provides a clean, publicly exposed Smithy-based SDK generation pipeline (`sdk-codegen`, `smithy-aws-typescript-codegen`), while an equivalent Java codegen/plugin workflow is not publicly available.

We can successfully generate Smithy AST JSON models and projections, but there is no documented/public Java Smithy client codegen plugin similar to JS v3. This makes it difficult to build custom AWS compatible SDK extensions, experiment with custom auth schemes, or generate Java SDK clients from Smithy models using a modern plugin-based workflow.

Having a publicly supported Java Smithy codegen pipeline would significantly improve extensibility, consistency across AWS SDK ecosystems, and adoption of Smithy for Java-based SDK development.

### Proposed Solution
Expose or document a public Smithy-based Java SDK code generation pipeline similar to AWS SDK JS v3.

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS Java SDK version used

2.31.22

### JDK version used

1.8

### Operating System and version

macOS 26.3 25D125