CMP0135 needs to apply to FetchContent as well

Policy CMP0135 added by !7137 (merged) was focused on preserving backward compatibility for ExternalProject, but the same concerns apply to FetchContent as well, since it uses a subset of the same set of keywords. FetchContent_Declare() needs to be updated to also consider the CMP0135 policy setting. Additionally, with the new behavior also recently added by !7276 (merged), the DOWNLOAD_EXTRACT_TIMESTAMP keyword will need to be added explicitly for the details forwarded to dependency providers when the URL download method is used.