Add a test to confirm that extensions are lazily decoded when we expect.
Drop the UnmarshalDefaultResolver flag. I added it thinking for some
reason that internal/impl couldn't depend on protoregistry; since it can
(and does), it's simpler to just test if the resolver is the expected
value.
Use a default set of options when lazily unmarshaling extensions.
Change-Id: Ied7666ffdc3bf90630260a80c9568d9a945048bc
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/218038
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
