Best effort fix errors while exporting objects

[certxlm]

Export-Clixml sometimes fails to exports all (gc)ADobjects, due to invalid encoded characters in some attributes.

Error observed usually during such events:

Error while exporting objects retrieved via LDAP '�', hexadecimal value 0xFFFF, is an invalid character

We end up with a truncated XML files, here is an example of such files:

<Objs Version="1.1.0.1" xmlns="http://schemas.microsoft.com/powershell/2004/04">
  <Obj RefId="0">
    <TN RefId="0">
[...]
 <Obj N="msExchAssistantsMaintenanceSchedule" RefId="13601">
        <TNRef RefId="1" />
        <LST>
          <S> <----- end of file

This does not seem to impact ingestion into splunk (maybe because extraction is done via regex?).

It is "kind of easily" reproductible by replacing a random attribute of such object with the following value:
$adobjects[0].random_attribute="should-not-be-$([char]0xffff)-exportable"

Nonetheless, it would be nice to exclude un-exportable objects and retry afterwards. The following PR uses the following static function to detect such objects:
[System.Management.Automation.PSSerializer]::Serialize

Note that it is a best-effort fix only, and has limitation (PSSerializer does not allow to set the encoding as in current code (UTF8), and has a default depth of 1 instead of 2 for export-clixml