OvmfPkg: QemuFwCfgLib: implement for SEC

OvmfPkg: QemuFwCfgLib: implement for SEC

The QemuFwCfgSecLib library instance
- is stateless,
- has no library constructor,
- is available to SEC client code,
- must be queried with QemuFwCfgIsAvailable() before use,
- is restricted to SEC in order to limit the explicit querying
  requirement. (There is no current user.)

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15046 6f19259b-4bc3-4df7-8a09-765794883524