Poor UX when attempting to apply http-types typed headers

This is not very nice:

```rust
let authz = BasicAuth::new("any", &api_key);
let res = client.put(&path)
.header(authz.name(), authz.value())
```

(From https://github.com/squamishaccess/squamishaccess-signup-function-rs/blob/6c445916760a4db21da80c732fbe8e9c704c1bd1/src/main.rs#L168-L172)

I think something like `ToHeaderPair` may be ideal? We could implement that for a tuple and for typed headers.
```rust
let authz = BasicAuth::new("any", &api_key);
let res = client.put(&path)
.header(authz)
// OR
.header((authz.name(), authz.value()))
```

This would be different than [`ToHeaderValues`](https://docs.rs/http-types/2.5.0/http_types/headers/trait.ToHeaderValues.html) since it would also have the header name.

@yoshuawuyts thoughts?

Start by reviewing the client's .header() API and the existing http-types ToHeaderValues trait referenced in the issue. Compare the proposed typed-header and tuple forms, then confirm that both can supply a header name and value without the current name()/value() calls; add or update coverage where the repository's header API tests live.

Written by the indexing model from the issue text.