feature(settings): add TLS (#380)

* Complete the missing TLS feature.

* Make the `cfg` attributes more clear.

* Format the project issued by command `cargo +nightly fmt`.

* Small changes on cargo file.

* Update CHANGES.md.

* Add documentation for `Tls::get_ssl_acceptor_builder()` and remove unused imports.

* Add the `cfg` macro with required feature on `TLS` tests.

* Update actix-settings/src/settings/tls.rs

Co-authored-by: Rob Ede <robjtede@icloud.com>

* Copy the workflow steps related to OpenSSL for windows from [actix-web workflow](a7375b6876/.github/workflows/ci.yml (L38-L45)).

* ci: install openssl 1.1.1

* Replaced `apply_settings` with `try_apply_settings` for a better error handling.

* Updated the example.

* Add `OpenSSL` error.

* Restrict `OpenSSL` error only for `tls` feature.

* Rename feature `tls` to `openssl`.

* Add doc feature `broken_intra_doc_links` to `get_ssl_acceptor_builder` function.

---------

Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-settings/examples/actix.rs

@@ -57,7 +57,7 @@ async fn main() -> std::io::Result<()> {
         }
     })
     // apply the `Settings` to Actix Web's `HttpServer`
-    .apply_settings(&settings)
+    .try_apply_settings(&settings)?
     .run()
     .await
 }