This adds an override mechanism for specifying alternative proto.Rewriter's when performing template rewrites. These new rules can be provided as an optional set of parameters to proto.ParseRewriteTemplate, so this does not change the calling signature when rules are not used. When provided, these rules are matched against the field name in a nested fashion (see the tests for an example).

We have some cases where we need to rewrite flags that are encoded as a single integer bit mask. By combining these rules with the new proto.BitOr type, we can merge flags instead of replacing them:

type Message struct {
	Flags uint64 `protobuf:"varint,2,opt,name=flags,proto3"`
}

typ := proto.TypeOf(reflect.TypeOf(Message{}))
template := []byte(`{"flags": 8}`) // n |= 0b1000
rw, err := proto.ParseRewriteTemplate(typ, template, proto.RewriterRules {
	"flags": proto.BitOr[uint64]{},
})