[core-util] depend on @typespec/ts-http-runtime

[timovv]

Packages impacted by this PR

• @azure/core-util
• @typespec/ts-http-runtime

Issues associated with this PR

• [core] Deduplicate code between @azure/core-* and @typespec/ts-http-runtime #32698

Describe the problem that is addressed by this PR

Opening this PR to get initial feedback on the approach, not looking to get it merged immediately. Let me know your thoughts.

This is a proof of concept of how we might deduplicate the Azure and unbranded package without changing the Azure API surface in any meaningful way. This is done by exporting the necessary internals from the unbranded package under a subpath export, __internal/<package-name>. The Azure package then wraps these internals and re-exports them. All the internals exported are prefixed with a double underscore to prevent a naming conflict when they are imported by the Azure package, and also to help make sure that none of the internal names show up in the public API of the Azure package.

Exporting under the __internal/<x> subpath keeps the main API surface of @typespec/ts-http-runtime clean and focused on the library's core purpose, which is to provide the necessary APIs for code generated from TypeSpec. This necessary surface is much smaller than what the combined Azure Core packages expose today.
 
I'm starting with core-util since it's one of the simpler packages to convert. This is because:

• there are no differences between Azure and unbranded, and
• it has no complex dependencies between Core packages (like for example core-client-rest has a dependency on core-rest-pipeline).

Other packages are a bit trickier but I've had some initial success with this approach even with these differences.

(more detailed description to come; let me know if any questions!)

[azure-sdk]

API change check

API changes are not detected in this pull request.