add Chapter 15 - runtime observer implementation

Chapter15/observer/libs/etl/appveyor.yml

@@ -0,0 +1,26 @@
+version: 1.0.{build}
+branches:
+  only:
+  - master
+image: Visual Studio 2022
+configuration:
+- Debug MSVC C++14
+- Debug MSVC C++14 - No STL
+- Debug MSVC C++17
+- Debug MSVC C++17 - No STL
+- Debug MSVC C++20
+- Debug MSVC C++20 - No STL
+clone_folder: c:\projects\etl
+install:
+- cmd: git submodule update --init --recursive
+build:
+  project: test/vs2022/etl.vcxproj
+  verbosity: minimal
+notifications:
+- provider: Webhook
+  url: https://hooks.slack.com/services/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
+  method: POST
+  on_build_success: true
+  on_build_failure: true
+  on_build_status_changed: true
+