Alerting: Add provisioning GET routes for message templates (#48367)

* Template service

* Add GET routes and implement them

* Generate mock for persist layer

* Unit tests for reading templates

* Set up composition root and get integration tests working

* Fix prealloc issue

* Extract setup boilerplate

* Update AuthorizationTest

* Rebase and resolve

* Fix linter error

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

@@ -2868,6 +2868,7 @@
    "x-go-package": "github.com/prometheus/alertmanager/timeinterval"
   },
   "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.",
    "properties": {
     "ForceQuery": {
      "type": "boolean"
@@ -2900,9 +2901,9 @@
      "$ref": "#/definitions/Userinfo"
     }
    },
-   "title": "URL is a custom URL type that allows validation at configuration load time.",
+   "title": "A URL represents a parsed URL (technically, a URI reference).",
    "type": "object",
-   "x-go-package": "github.com/prometheus/common/config"
+   "x-go-package": "net/url"
   },
   "Userinfo": {
    "description": "The Userinfo type is an immutable encapsulation of username and\npassword details for a URL. An existing Userinfo value is guaranteed\nto have a username set (potentially empty, as allowed by RFC 2396),\nand optionally a password.",
@@ -3553,7 +3554,6 @@
    "x-go-package": "github.com/prometheus/alertmanager/api/v2/models"
   },
   "receiver": {
-   "description": "Receiver receiver",
    "properties": {
     "name": {
      "description": "name",
@@ -3564,7 +3564,9 @@
    "required": [
     "name"
    ],
-   "type": "object"
+   "type": "object",
+   "x-go-name": "Receiver",
+   "x-go-package": "github.com/prometheus/alertmanager/api/v2/models"
   },
   "silence": {
    "description": "Silence silence",
@@ -4956,6 +4958,43 @@
     ]
    }
   },
+  "/api/provisioning/templates": {
+   "get": {
+    "operationId": "RouteGetTemplates",
+    "responses": {
+     "200": {
+      "$ref": "#/responses/MessageTemplate"
+     },
+     "400": {
+      "description": "ValidationError",
+      "schema": {
+       "$ref": "#/definitions/ValidationError"
+      }
+     }
+    },
+    "summary": "Get all message templates.",
+    "tags": [
+     "provisioning"
+    ]
+   }
+  },
+  "/api/provisioning/templates/{ID}": {
+   "get": {
+    "operationId": "RouteGetTemplate",
+    "responses": {
+     "200": {
+      "$ref": "#/responses/MessageTemplate"
+     },
+     "404": {
+      "$ref": "#/responses/NotFound"
+     }
+    },
+    "summary": "Get a message template.",
+    "tags": [
+     "provisioning"
+    ]
+   }
+  },
   "/api/ruler/grafana/api/v1/rules": {
    "get": {
     "description": "List rule groups",