extension_request and custom_attribute should be pluralised

[jessereynolds]

It was surprising to me that the parameter for specifying multiple certificate extension requests via an array was the singular extension_request, and similarly for custom_attribute. We talk about "extension requests" in the docs, we have parameters in the install.ps1 and install.bash scripts named extension_requests and custom_attributes, so it makes sense to be consistent.

Recommendation: rename these parameters to extension_requests and custom_attributes. Possibly add aliases of these for the singular versions for backwards compatibility, although the module is not yet at 1.0 so this isn't strictly necessary.