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/spec.json

@@ -2828,11 +2828,6 @@
         }
       }
     },
-    "DsPermissionType": {
-      "description": "Datasource permission\nDescription:\n`0` - No Access\n`1` - Query\nEnum: 0,1",
-      "type": "integer",
-      "format": "int64"
-    },
     "Duration": {
       "type": "integer",
       "format": "int64",
@@ -3483,12 +3478,6 @@
     "GettableNGalertConfig": {
       "type": "object",
       "properties": {
-        "alertmanagers": {
-          "type": "array",
-          "items": {
-            "type": "string"
-          }
-        },
         "alertmanagersChoice": {
           "type": "string",
           "enum": [
@@ -4422,12 +4411,6 @@
     "PostableNGalertConfig": {
       "type": "object",
       "properties": {
-        "alertmanagers": {
-          "type": "array",
-          "items": {
-            "type": "string"
-          }
-        },
         "alertmanagersChoice": {
           "type": "string",
           "enum": [
@@ -5524,8 +5507,9 @@
       }
     },
     "URL": {
+      "description": "The general form represented is:\n\n[scheme:][//[userinfo@]host][/]path[?query][#fragment]\n\nURLs that do not start with a slash after the scheme are interpreted as:\n\nscheme:opaque[?query][#fragment]\n\nNote that the Path field is stored in decoded form: /%47%6f%2f becomes /Go/.\nA consequence is that it is impossible to tell which slashes in the Path were\nslashes in the raw URL and which were %2f. This distinction is rarely important,\nbut when it is, the code should use RawPath, an optional field which only gets\nset if the default encoding is different from Path.\n\nURL's String method uses the EscapedPath method to obtain the path. See the\nEscapedPath method for more details.",
       "type": "object",
-      "title": "URL is a custom URL type that allows validation at configuration load time.",
+      "title": "A URL represents a parsed URL (technically, a URI reference).",
       "properties": {
         "ForceQuery": {
           "type": "boolean"
@@ -5536,9 +5520,6 @@
         "Host": {
           "type": "string"
         },
-        "OmitHost": {
-          "type": "boolean"
-        },
         "Opaque": {
           "type": "string"
         },
@@ -5740,6 +5721,7 @@
       "$ref": "#/definitions/alertGroup"
     },
     "alertGroups": {
+      "description": "AlertGroups alert groups",
       "type": "array",
       "items": {
         "$ref": "#/definitions/alertGroup"
@@ -5845,7 +5827,6 @@
       }
     },
     "gettableAlert": {
-      "description": "GettableAlert gettable alert",
       "type": "object",
       "required": [
         "labels",
@@ -5902,7 +5883,6 @@
       "$ref": "#/definitions/gettableAlert"
     },
     "gettableAlerts": {
-      "description": "GettableAlerts gettable alerts",
       "type": "array",
       "items": {
         "$ref": "#/definitions/gettableAlert"
@@ -5910,6 +5890,7 @@
       "$ref": "#/definitions/gettableAlerts"
     },
     "gettableSilence": {
+      "description": "GettableSilence gettable silence",
       "type": "object",
       "required": [
         "comment",
@@ -5959,7 +5940,6 @@
       "$ref": "#/definitions/gettableSilence"
     },
     "gettableSilences": {
-      "description": "GettableSilences gettable silences",
       "type": "array",
       "items": {
         "$ref": "#/definitions/gettableSilence"
@@ -5967,7 +5947,6 @@
       "$ref": "#/definitions/gettableSilences"
     },
     "integration": {
-      "description": "Integration integration",
       "type": "object",
       "required": [
         "name",
@@ -6112,6 +6091,7 @@
       }
     },
     "postableSilence": {
+      "description": "PostableSilence postable silence",
       "type": "object",
       "required": [
         "comment",