Invalid cookies can be created without error

[Cookie::set_value](https://docs.rs/actix-web/latest/actix_web/cookie/struct.Cookie.html#method.set_value) accepts any unicode string and never fails, but some unicode strings [are not valid cookie values](https://datatracker.ietf.org/doc/html/rfc6265#section-4.1.1).

Could we add a `try_set_value` method that only accepts valid values ?

Otherwise, we end up with confusing error messages, generated far from where the true source of the error is.

Start with Cookie::set_value in the linked API documentation and compare accepted values with RFC 6265 §4.1.1. Done means a try_set_value entry point rejects invalid cookie values and exposes the failure at the call site, with coverage for accepted and rejected values.

索引モデルが issue の本文から書いたものです。