move to using Arc<dyn HttpClient> in azure_identity

[bmc-msft]

From discussions in a previous PR, the more common method in the SDK is to clone the Arc<dyn HttpClient>, rather than pass a reference.

#785 (comment)

Note, this uses the crate fix-hidden-lifetime-bug to work around an issue with async fn trait issues (ref: rust-lang/rust#63033).

Without this workaround, we see the error:

error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
  --> sdk/identity/src/client_credentials_flow/mod.rs:59:28
   |
59 |   ) -> Result<LoginResponse> {
   |  ____________________________^
60 | |     let encoded: String = form_urlencoded::Serializer::new(String::new())
61 | |         .append_pair("client_id", client_id.as_str())
62 | |         .append_pair("scope", &scopes.join(" "))
...  |
89 | |     serde_json::from_slice(&rsp_body).map_kind(ErrorKind::DataConversion)
90 | | }
   | |_^
   |
   = note: hidden type `impl futures::Future<Output = std::result::Result<client_credentials_flow::login_response::LoginResponse, azure_core::error::Error>>` captures lifetime '_#42r

[cataggar]

😱