Expose response headers in service result as a Pair

[darronschall]

In creating #2, we allowed for sending arbitrary headers, such an ETag value for If-None-Match, or a value for Idempotency-Key.

This PR provides a way to read headers from the service result. The only way to capture the returned ETag value is to read the response headers, which was impossible prior to this PR.

There's also a small consistency change. Since we're using the Headers type in the result for the response, we change the request type to also be Headers to match. This changes the call site a bit since the headers should go through HeadersBuilder:

val requestHeaders = eTag?.let { Headers.build { append(HttpHeaders.IfNoneMatch, eTag) } }

Or, if sending multiple headers, something like:

val requestHeadersBuilder: HeadersBuilder = HeadersBuilder()
eTag?.let { requestHeadersBuilder.append(HttpHeaders.IfNoneMatch, it) }
// Could add more headers here as necessary
val requestHeaders = requestHeadersBuilder.build()

On the response side, use destructuring to get at the pair values:

val (response, responseHeaders) = exampleService.doSomethingOrOther(...)

... or, when you don't care about the response headers:

val (response, _) = exampleService.doSomething(...)