[Prototype] Migrate spec-update to shared reusable workflow

[cguldner]

Prototype — do not merge. Demonstrates consolidating the duplicated spec_update.yml into a shared reusable workflow.

• Replaces the 138-line spec_update.yml with an 18-line thin caller pointing at cguldner/sdk-build-tools/.github/workflows/spec-update.yml@master (prototype location; intended home dropbox/sdk-build-tools).
• Adds scripts/spec-update/generate.sh — the per-repo codegen (the only repo-specific part), runnable/testable locally.
• Net: −127 lines here; the shared envelope (AWS OIDC, app token, submodule diff, create-PR, auto-merge) lives once in the shared repo.

Before merging / rolling out

• Verify IAM role oidc-github-dropbox-dropbox-sdk-python-repo trust policy keys on caller sub (repo:dropbox/dropbox-sdk-python:*), not job_workflow_ref. If it pins job_workflow_ref, OIDC breaks until the policy is updated.
• Confirm org Actions policy allows the reusable workflow (repos use a selected allow-list).
• Test via workflow_dispatch on this branch before merge.

To test

Trigger the Spec Update workflow on branch prototype-reusable-spec-update via workflow_dispatch and confirm the AWS auth + token steps succeed and a spec-update PR is produced identical in shape to today's.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.18%. Comparing base (2151fb6) to head (7d60245).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #572   +/-   ##
=======================================
  Coverage   65.18%   65.18%           
=======================================
  Files          34       34           
  Lines       65830    65830           
  Branches     4687     4687           
=======================================
  Hits        42910    42910           
  Misses      22759    22759           
  Partials      161      161           

Flag	Coverage Δ
integration	65.04% <ø> (ø)
unit	64.80% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cguldner]

Closing for now — will revisit once dropbox/sdk-build-tools is created (the reusable workflow needs an org-hosted home; calling it from cguldner/ hit a startup_failure due to org Actions policy). Branch retained.