Add API to serve blob or LFS file content (#19689)

* Add LFS API

* Update routers/api/v1/repo/file.go

Co-authored-by: Gusted <williamzijl7@hotmail.com>

* Apply suggestions

* Apply suggestions

* Update routers/api/v1/repo/file.go

Co-authored-by: Gusted <williamzijl7@hotmail.com>

* Report errors

* ADd test

* Use own repo for test

* Use different repo name

* Improve handling

* Slight restructures

1. Avoid reading the blob data multiple times
2. Ensure that caching is only checked when about to serve the blob/lfs
3. Avoid nesting by returning early
4. Make log message a bit more clear
5. Ensure that the dataRc is closed by defer when passed to ServeData

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/swagger/v1_json.tmpl

@@ -7150,6 +7150,52 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/media/{filepath}": {
+      "get": {
+        "tags": [
+          "repository"
+        ],
+        "summary": "Get a file or it's LFS object from a repository",
+        "operationId": "repoGetRawFileOrLFS",
+        "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": "filepath of the file to get",
+            "name": "filepath",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "The name of the commit/branch/tag. Default the repository’s default branch (usually master)",
+            "name": "ref",
+            "in": "query"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Returns raw file content."
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/milestones": {
       "get": {
         "produces": [