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.

Written by the indexing model from the issue text.