Provide with_header method on client

[jongiddy]

This change allows arbitrary headers to be included in requests to ClickHouse.

It uses HeaderName and HeaderValue which is efficient for use with hyper. The types are provided by the http crate to allow support for other HTTP client crates to use these types without requiring the full hyper crate.

There are other ways to provide this capability, including unsealing the HttpClient trait to allow more implementations. However, this change provides a limited additional capability until a public interface for HttpClient is decided.

Fixes #98

[slvrtrn]

In the hyper documentation, they use plain strings as headers names/values: https://docs.rs/hyper/latest/hyper/struct.Request.html#examples

It uses HeaderName and HeaderValue which is efficient for use with hyper.

If there are optimizations made by HeaderName, and the custom headers will likely use some non-standard names (X-My-Proxy-Auth and similar), does it make sense to add http as a dependency, or maybe we can get away with just &str?

From https://docs.rs/http/latest/http/header/struct.HeaderName.html#representation:

HeaderName represents standard header names using an enum, as such they will not require an allocation for storage. All custom header names are lower cased upon conversion to a HeaderName value.

As I understand it, the custom header names are not represented as enums.

[loyd]

LGTM, let's also pass to INSERTs

[loyd]

I think we can accept impl TryInto to make it possible to pass strings. Downside here is that we cannot use HeaderMap and should use HashMap<Result<_>, Result<_>> instead =(

[CLAassistant]

All committers have signed the CLA.

[jongiddy]

I rebased the code on the latest version and rewrote it to use plain Strings without requiring the http crate. The client headers now follow a similar structure to client options.

[loyd]

LGTM, but need to fix rustfmt warnings

[loyd]

Merged, thanks for your contribution!