Add nox_helpers.py to the monorepo root.

### Summary of the feature request

Consolidate package level `noxfile.py` file contents into a single global file at the monorepo root level.

**Current State**: 30 packages handwritten or hybrid packages define their own noxfile.py.
**Similarity Profile**: Average similarity is 33.4%, but this is due to package names and line counts ranging from 186 to 1085.
**Why it's a prime candidate**: Although the files appear diverse, the actual logic for standard nox sessions (`lint`, `blacken`, `mypy`, `docs`, `docfx`) is heavily duplicated boilerplate generated from common Jinja templates (e.g., `gapic/templates/noxfile.py.j2`).

**Recommendation**:
Create a shared `nox_helpers.py` utility module in the project root containing standard session definitions.
Refactor local per-package noxfile.py files to import and call these shared utilities, passing only package-specific variables (like package path and test requirements).

This can reduce each local noxfile from ~565 lines to under 30 lines.