[REQ][Rust] Encode parameters in URL with less allocation

Is your feature request related to a problem? Please describe.

The parameters to each API call are passed into the client function as type &str but are then converted to String by the URL encoding call using the url::form_urlencoded crate. These newly allocated Strings are then passed to the format! macro to create the API URL.

pub async fn list_project_types(
    configuration: &configuration::Configuration, project: &str
) -> Result<crate::models::ProjectTypesListResponse, Error<ListProjectTypesError>> {
    ...
    let local_var_uri_str = format!(
        "{}/project/{project}/types",
        local_var_configuration.base_path,
        project=crate::apis::urlencode(project)  // (&str) -> String
    );
    ...

This requires n+1 allocations for n parameters.

Describe the solution you'd like

The form_urlencoded crate depends on the percent-encoding crate. Using the percent-encoding crate directly for the generated urlencode function allows parameters to be returned as type Cow, avoiding allocation for parameters that do not require any percent encoding.

The existing function

pub fn urlencode<T: AsRef<str>>(s: T) -> String {
    ::url::form_urlencoded::byte_serialize(s.as_ref().as_bytes()).collect()
}

can be replaced with:

pub fn urlencode<T: AsRef<str>>(s: T) -> Cow<str> {
    ::percent_encoding::utf8_percent_encode(s.as_ref(), &AN_ASCII_SET).into()
}

Note: I've skipped the lifetime annotations and definition of AN_ASCII_SET that would be required for a PR.

Describe alternatives you've considered

We could continue to use form_urlencoded but replace the collect with code duplicated from percent-encoding to generate the Cow from an iterator.

Additional context

Start at the generated Rust client's urlencode function and inspect how the Rust generator declares and uses URL-encoding dependencies. Replace the allocation-heavy encoding path with the requested Cow<str> approach, then verify that generated client URLs remain correctly encoded and avoid allocation when parameters need no percent encoding.