Get a plugin's release notes

In whichever form the plugin declared via releaseNotesStyle in its pluginInfo.json (see the Template's PLUGININFO_FORMAT.md). Installed plugins only; the style is read from the pluginInfo.json the next update would land, else the installed one. - gitRelease: the latest GitHub Release of the srcURL repo (GitHub only; offered as none otherwise), proxied and cached for 6h, using the configured GitHub token when there is one. release is null when the repo has no Release; stale is true when GitHub could not be reached and an older cached copy is served. - gitHistory: pending is what Update would pull in (HEAD..origin/<branch>, or HEAD..the versions[] pin for a pinned plugin, as of the last update check), installed the most recent installed commits. Both are empty-safe; being up to date is not an error. - script: the stdout of the plugin's installed scripts/fpp_releasenotes.sh (15s timeout, 64 KiB cap), as plain text; offered as none until the script is installed. 404 when the plugin is not installed, declares no release notes, or its repo isn't visible to GitHub's API; 502 when GitHub or the plugin's script failed. Errors carry Status/Message.