[Python O11Y] Fix core header dependency issue (#35845)

Core team [moved resolver code](https://github.com/grpc/grpc/pull/35804) to `src/core/resolver`, our build is failing because we have a dependency on it and the resolver folder is not copied to `observability/grpc_root`:
 * `call_tracer.h` -> `core_configuration.h` -> `lb_policy_registry.h` -> `lb_policy.h` -> `endpoint_addresses.h`

To fix this issue and to prevent such issues from happening again, instead of subfolders, we'll copy all files from `src/core` to `observability/grpc_root/src/core`.

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Closes #35845

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35845 from XuanWang-Amos:fix_o11y_core_header_deps 3c9eb8c499
PiperOrigin-RevId: 605063207

src/python/grpcio_observability/make_grpcio_observability.py

@@ -66,11 +66,7 @@ GRPCIO_OBSERVABILITY_ROOT_PREFIX = "src/python/grpcio_observability/"
 COPY_FILES_SOURCE_TARGET_PAIRS = [
     ("include", "grpc_root/include"),
     ("third_party/abseil-cpp/absl", "third_party/abseil-cpp/absl"),
-    ("src/core/lib", "grpc_root/src/core/lib"),
+    ("src/core", "grpc_root/src/core"),
-    (
-        "src/core/load_balancing",
-        "grpc_root/src/core/load_balancing",
-    ),
 ]
 
 # grpc repo root