Expressions: Move GEL into core as expressions (#29072)

* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins

pkg/plugins/testdata/behind-feature-flag/gel/MANIFEST.txt

@@ -1,23 +0,0 @@
-
------BEGIN PGP SIGNED MESSAGE-----
-Hash: SHA512
-
-{
-  "plugin": "gel",
-  "version": "1.0.0",
-  "files": {
-    "plugin.json": "b9b3bb0dab3c4655a929a1e48a957466e3e2717992bdd29da27e5eed2fae090c"
-  },
-  "time": 1589274667427,
-  "keyId": "7e4d0c6a708866e7"
-}
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-Comment: https://openpgpjs.org
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pkg/plugins/testdata/behind-feature-flag/gel/plugin.json

@@ -1,14 +0,0 @@
-{
-  "type": "transform",
-  "name": "GEL",
-  "id": "gel",
-  "backend": true,
-  "info": {
-    "description": "Test",
-    "version": "1.0.0",
-    "author": {
-      "name": "Grafana Labs",
-      "url": "https://grafana.com"
-    }
-  }
-}