plugins: Introduce generated, static core plugin registry (#54118)

* Refactor towards template/codegen framework

* Add templates for plugin gen

* Add Go codegen for plugins; overhaul framework, too

* Add new codegen output; assorted framework fixes

* Regenerate after merge

* Remove accidental commit file, update templates

* Export the pfs.Tree loader from plugin types

* Print details from cuetsy errors

* Generate loaders for all plugins and list in registry

* Use pfs_gen.go over lineage_gen.go

* Un-un-ignore main file

* Introduce simple List static registry for plugins

* Last tweaks to codegen

* remove unused tvars

* Ensure loop-local instances for both vars

* Generate pfs parsing in-place in registry

* Stop generating pfs_gen.go

* Move Tree into pfs, rename subdir

* Change package name to match dir

* Ignore gocyclo on HTTPServer.getNavTree

pkg/api/index.go

@@ -173,6 +173,7 @@ func (hs *HTTPServer) ReqCanAdminTeams(c *models.ReqContext) bool {
 	return c.OrgRole == org.RoleAdmin || (hs.Cfg.EditorsCanAdmin && c.OrgRole == org.RoleEditor)
 }
 
+//nolint:gocyclo
 func (hs *HTTPServer) getNavTree(c *models.ReqContext, hasEditPerm bool, prefs *pref.Preference) ([]*dtos.NavLink, error) {
 	hasAccess := ac.HasAccess(hs.AccessControl, c)
 	var navTree []*dtos.NavLink