net.box: refactor netbox method encoders arguments

In scope of #8633 call and eval encoding will require an additional
argument (whether to encode tuples as extensions and send formats), and
method encoders already have 3 arguments (MsgPack stream, IPROTO sync and
IPROTO stream identifier): to simplify further extension of method encoders
move arguments to a netbox_method_encode_ctx struct.

Needed for #8633

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