Break out serializer into its own cpp file

This commit is entirely just moving code, such that not all compile
units need to pull in the full html serializer.

Tested: Unit tests pass.  Pretty good coverage.

Redfish service validator passes.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ifaebe9534c0693dc678fd994517563b89aca0cc5
diff --git a/meson.build b/meson.build
index 6cdde78..cdfc3dd 100644
--- a/meson.build
+++ b/meson.build
@@ -338,6 +338,7 @@
   'src/boost_beast.cpp',
   'src/boost_url.cpp',
   'src/dbus_singleton.cpp',
+  'src/json_html_serializer.cpp',
 )
 
 bmcweblib = static_library(