Add `SerializedMessageReader2`. It replaces `SerializedMessageReader` which will
be removed.

Summary of differences between `SerializedMessageReader` and
`SerializedMessageReader2`:

* The set of field handlers is a compile time parameter list.

* Field numbers are compile time constants.

* Field dispatch is much faster thanks to the above.

* The `Context` template parameter is variadic. The `context` arguments must be
  lvalue references.

* Natively parsed submessages are handled differently, avoiding moving message
  objects.

* Processing nested fields involves nested function calls, rather than
  specifying field paths.

* `SerializedMessageReader2` is not actually a class but a factory function.

* Most field handlers do not get direct access to the `Reader`.

`SerializedMessageWriter` and `SerializedMessageBackwardWriter` stay unchanged.
`SerializedMessageRewriter` will be removed without a dedicated replacement.

A cosmetic change in `SerializedMessageWriter`: pass arguments of
`LengthOfString()` by const reference (they are not more efficient when moved,
and it is likely a bug to `std::move` them when they are later written).

PiperOrigin-RevId: 829458416