No Ability for Admin to Override Configs or Scripts to Ensure Special Spec PR Following Review Workflow

Special case: A service team wants to add 3P npm package to generate sdk from tsp. But the npm package dependency is fixed: [azure-sdk-for-net/eng/emitter-package.json at main · Azure/azure-sdk-for-net](https://github.com/Azure/azure-sdk-for-net/blob/main/eng/emitter-package.json). It force thie team to generate SDK locally. It causes at least following issues:
- sdk automation always fails in codegen stage.
- They can't follow spec repo's review workflow, we cannot verify anything to ensure the generated package works and has no unapproved breaking changes.
- Running locally for all languages takes a lot of time and may encounter unexpected errors.

My proposal to reuse sdk automation pipeline as much as possible and make special spec PR follow Spec review workflow as much as possible:
Make configs and scripts overriadable by pipeline variables by admin (subset azure sdk team, maybe?)

Start by reviewing the SDK automation pipeline and the referenced eng/emitter-package.json dependency configuration. Determine which configs and scripts must accept administrator-controlled pipeline overrides for the special spec PR workflow. Done means the workflow can use the required package without local generation while preserving review and validation checks.

Written by the indexing model from the issue text.