Add "partial" to RequestInit definition

[tidoust]

The prose is clear that the IDL definition of RequestInit needs to be adjusted in Fetch to add targetAddressSpace and that the IDL should be read as a partial definition, but tools that process IDL in specs automatically (typically Reffy to create the Webref database) don't read prose and see two definitions of RequestInit, one in Fetch and on in Private Network Access.

This update adds the partial keyword to make it clear to spec IDL scrappers that Private Network Access extends the definition of RequestInit.

[letitz]

Thanks for catching and sending this in!