Alerting: Remove url based external alertmanagers config (#57918)

* Remove URL-based alertmanagers from endpoint config

* WIP

* Add migration and alertmanagers from admin_configuration

* Empty comment removed

* set BasicAuth true when user is present in url

* Remove Alertmanagers from GET /admin_config payload

* Remove URL-based alertmanager configuration from UI

* Fix new uid generation in external alertmanagers migration

* Fix tests for URL-based external alertmanagers

* Fix API tests

* Add more tests, move migration code to separate file, and remove possible am duplicate urls

* Fix edge cases in migration

* Fix imports

* Remove useless fields and fix created_at/updated_at retrieval

Co-authored-by: George Robinson <george.robinson@grafana.com>
Co-authored-by: Konrad Lalik <konrad.lalik@grafana.com>

pkg/services/ngalert/api/tooling/api.json

@@ -395,11 +395,6 @@
    ],
    "type": "object"
   },
-  "DsPermissionType": {
-   "description": "Datasource permission\nDescription:\n`0` - No Access\n`1` - Query\nEnum: 0,1",
-   "format": "int64",
-   "type": "integer"
-  },
   "Duration": {
    "format": "int64",
    "title": "Duration is a type used for marshalling durations.",
@@ -1046,12 +1041,6 @@
   },
   "GettableNGalertConfig": {
    "properties": {
-    "alertmanagers": {
-     "items": {
-      "type": "string"
-     },
-     "type": "array"
-    },
     "alertmanagersChoice": {
      "enum": [
       "all",
@@ -1984,12 +1973,6 @@
   },
   "PostableNGalertConfig": {
    "properties": {
-    "alertmanagers": {
-     "items": {
-      "type": "string"
-     },
-     "type": "array"
-    },
     "alertmanagersChoice": {
      "enum": [
       "all",
@@ -3097,9 +3080,6 @@
     "Host": {
      "type": "string"
     },
-    "OmitHost": {
-     "type": "boolean"
-    },
     "Opaque": {
      "type": "string"
     },
@@ -3301,7 +3281,6 @@
    "type": "object"
   },
   "alertGroups": {
-   "description": "AlertGroups alert groups",
    "items": {
     "$ref": "#/definitions/alertGroup"
    },
@@ -3462,7 +3441,6 @@
    "type": "object"
   },
   "gettableAlerts": {
-   "description": "GettableAlerts gettable alerts",
    "items": {
     "$ref": "#/definitions/gettableAlert"
    },
@@ -3667,6 +3645,7 @@
    "type": "array"
   },
   "postableSilence": {
+   "description": "PostableSilence postable silence",
    "properties": {
     "comment": {
      "description": "comment",