Add API for changing Avatars (#25369)

This adds an API for uploading and Deleting Avatars for of Users, Repos
and Organisations. I'm not sure, if this should also be added to the
Admin API.

Resolves #25344

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/structs/user.go

@@ -102,3 +102,9 @@ type RenameUserOption struct {
 	// unique: true
 	NewName string `json:"new_username" binding:"Required"`
 }
+
+// UpdateUserAvatarUserOption options when updating the user avatar
+type UpdateUserAvatarOption struct {
+	// image must be base64 encoded
+	Image string `json:"image" binding:"Required"`
+}