Add HTTPClientHandler to BetterstackHttpClient

[pedrobarrios-png]

This PR introduces an optional HttpClientHandler into the BetterStack HTTP client and exposes it through the LoggerSinkConfigurationExtensions BetterStack sink configuration.
Mainly to allow callers to supply a custom HttpClientHandler (for proxy settings, client certificates, custom SocketsHttpHandler options, etc).

BetterStackHttpClient.cs

• Added an optional constructor parameter: HttpClientHandler? httpClientHandler = null
• If a handler is provided, the internal HttpClient is constructed with it new HttpClient(httpClientHandler); otherwise a default HttpClient() is used.

LoggerSinkConfigurationExtensions.cs

• Added an optional parameter to the BetterStack sink extension: HttpClientHandler? httpClientHandler = null

Usage example
Passing a custom handler to the sink:
var handler = new HttpClientHandler { Proxy, UseProxy, ClientCertificates, etc };
Log.Logger = new LoggerConfiguration().WriteTo.BetterStack(sourceToken, httpClientHandler: handler).CreateLogger();

[PetrHeinz]

Thanks @pedrobarrios-png for your contribution 🙌

The updated version works nicely, and provides nice additional options of configurations with full backward compatibility 🚀

I'll make sure to release as v1.2.0.