Remove magic numbers

There's lots of magic numbers in this file that we inherited from crow.
This commit Adds a TypeCode enum class that can be used in place of the
magic numbers.  It keeps the same values as were present previously
(0-6) in case there are places where this abstraction leaked out, but I
believe this catches all of them.

Tested: Redfish service validator passes.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I063955adb8bf75d9bb6298e29e6e44c210ee9cc3