uSync: added release workflow, cut v17.4.0

Repo: KevinJump/uSync

v17 had no tag-triggered NuGet release workflow (only nightly-feed packaging and PR build/test) — ported release.yml from v18/main, adjusted for v17's unpinned global.json (PR #1080)
Cut a test release v17.4.0-alpha to prove the pipeline end to end (tag → build → pack → NuGet trusted publishing), then kept it as a real prerelease on NuGet
Released v17.4.0 — background processing mode (opt-in, with load-balanced backoffice support), a structured diff view for JSON/block property changes, revived uSync.Community.DataTypeSerializers, and cross-platform path/filename fixes