Refactor "Content" for file uploading (#25851)

Before: the concept "Content string" is used everywhere. It has some
problems:

1. Sometimes it means "base64 encoded content", sometimes it means "raw
binary content"
2. It doesn't work with large files, eg: uploading a 1G LFS file would
make Gitea process OOM

This PR does the refactoring: use "ContentReader" / "ContentBase64"
instead of "Content"

This PR is not breaking because the key in API JSON is still "content":
`` ContentBase64 string `json:"content"` ``

tests/integration/api_repo_file_helpers.go

@@ -4,6 +4,8 @@
 package integration
 
 import (
+	"strings"
+
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
@@ -15,9 +17,9 @@ func createFileInBranch(user *user_model.User, repo *repo_model.Repository, tree
 	opts := &files_service.ChangeRepoFilesOptions{
 		Files: []*files_service.ChangeRepoFile{
 			{
-				Operation: "create",
-				TreePath:  treePath,
-				Content:   content,
+				Operation:     "create",
+				TreePath:      treePath,
+				ContentReader: strings.NewReader(content),
 			},
 		},
 		OldBranch: branchName,