Refactor: break up event_service_manager.hpp

'class Subscription' can be extracted into a separate file.

No changes have been made to the code.

Tested:

- Using Redfish Event Listener, test subscriptions and eventing.
- Redfish Service Validator passes

Change-Id: Id0076ef617e36cbb85629a386a4511a4fdb5e4da
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/meson.build b/meson.build
index fe6f113..af2c69c 100644
--- a/meson.build
+++ b/meson.build
@@ -346,6 +346,7 @@
     'redfish-core/src/filter_expr_printer.cpp',
     'redfish-core/src/redfish.cpp',
     'redfish-core/src/registries.cpp',
+    'redfish-core/src/subscription.cpp',
     'redfish-core/src/utils/dbus_utils.cpp',
     'redfish-core/src/utils/json_utils.cpp',
     'redfish-core/src/utils/time_utils.cpp',