API: Get release by tags endpoint (#12932)

Get a release based on a tag name (for which a release exists).
Based on:
https://developer.github.com/v3/repos/releases/#get-a-release-by-tag-name

Co-authored-by: 赵智超 <1012112796@qq.com>
Co-authored-by: 6543 <6543@obermui.de>

templates/swagger/v1_json.tmpl

@@ -7685,6 +7685,49 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/releases/tags/{tag}": {
+      "get": {
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Get a release by tag name",
+        "operationId": "repoGetReleaseTag",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "tagname of the release to get",
+            "name": "tag",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/responses/Release"
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/releases/{id}": {
       "get": {
         "produces": [