fix(utoipa-gen): remove unnecessary allocation with to_string in expanded code

[CBenoit]

Taking something that is a string and converting it to a string using to_string() kind of misses the point of why we are doing the conversion in the first place, and also fails to document this to the reader. to_owned() fully captures the reason that a conversion is required at a particular spot: borrowed (&str) to owned (String).

Some people enable this clippy lint in order to enforce this style:

- https://rust-lang.github.io/rust-clippy/master/index.html#/str_to_string

Code generated by utoipa macro is triggering this lint. To avoid that, this patch is replacing the to_string() usages in generated code by to_owned().

This patch is not changing any other to_string() because the utoipa project itself does not need to adhere to this style. The goal is just to reduce friction for downstream consumers.

Here is how it shows up without this fix:

warning: `to_string()` called on a `&str`
   --> devolutions-gateway/src/api/webapp.rs:271:1
    |
271 | / #[utoipa::path(
272 | |     post,
273 | |     operation_id = "SignSessionToken",
274 | |     tag = "WebApp",
...   |
286 | |     ),
287 | | )]
    | |__^
    |
    = help: consider using `.to_owned()`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#str_to_string
    = note: this warning originates in the attribute macro `utoipa::path` (in Nightly builds, run with -Z macro-backtrace for more info)

There is no lint for the opposite ("use to_string() instead of to_owned()), so it seems safe to apply this fix.

[juhaku]

Might be just useful to try not to actually use the .to_string() or to_owned() call in the first place. Mist likely it is relic from older times. But anyhow if it is necessary to have either one then I guess this change for the to_owned() is alright.

[CBenoit]

Sounds right! I removed the superfluous allocation.

[juhaku]

Great 💯