Expand configuration options of page export

[nils-a]

Coming from pnp/powershell#880

I'd like to be able to export page templates only.

My first idea would be, since the ObjectClientSidePageContents handles those templates fine already, to extend the ProvisioningTemplateCreationInformation:
Currently, ProvisioningTemplateCreationInformation contains the IncludeAllClientSidePages-Property which determines whether all pages are exported, or only the "home page".

A new property ProvisioningTemplateCreationInformation.IncludeClientSidePageTemplates could be used to determine whether templates should be exported.
Alternatively, something akin to the existing handlers could be used: An enum to distinguish between different page "types". This option would have the benefit of being open for additional changes, without cluttering the ProvisioningTemplateCreationInformation too much.