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/codegen/tmpl/autogen_header.tmpl

@@ -0,0 +1,28 @@
+{{ if .GenLicense -}}
+// Copyright {{ now.Year }} Grafana Labs
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+{{ end -}}
+// This file is autogenerated. DO NOT EDIT.
+{{- if ne .GeneratorPath "" }}
+//
+// Generated by {{ .GeneratorPath }}
+{{- end }}
+{{- if ne .LineagePath "" }}
+//
+// Derived from the Thema lineage declared in {{ .LineagePath }}{{ if ne .LineageCUEPath "" }} at CUE path "{{ .LineageCUEPath }}"{{ end }}
+{{- end }}
+//
+// Run `make gen-cue` from repository root to regenerate.
+