Issue #42: Added support for a custom HttpMessageHandler

[ElasticCoder]

This PR allows the consumer to specify a custom HttpMessageHandler.

This may be to:

1. override SSL verification and support connecting to the emulator from a Linux machine across the network, for example when connecting from within a Linux container
2. specifying an HTTP proxy if one is needed to connect to the Internet

A new constructor has been added to DocumentClient which has the same signature as the V2 SDK - it allows the consumer to inject an HttpMessageHandler which is then wired up to the InnerHandler of the HttpRequestMessageHandler.

The CosmosClientBuilder has a UseHttpMessageHandler method addded to it which allows the consumers to add the handler.

var handler = new HttpClientHandler();
handler.ServerCertificateCustomValidationCallback += (message, certificate2, x509Chain, sslPolicyErrors) => true;

using (var client = new CosmosClientBuilder(accountEndpoint, accountKey)
                    .UseConnectionModeGateway()
                    .UseHttpMessageHandler(handler)
                    .Build())
{
    // My code goes here
}

[ElasticCoder]

@kirankumarkolli I can't seem to add you as a reviewer, but please can you review this change also?

[ealsur]

@ElasticCoder We are going to move ahead with this (sorry for the delay), could you merge master into your fork? We can then proceed to verify and merge this

[ElasticCoder]

Hi, I've gone to look at this today, but it looks like the changes have been added to the master?
#136

[ausfeldt]

test

[ausfeldt]

/AzurePipelines run

[azure-pipelines]

Pull request contains merge conflicts.

[j82w]

@ElasticCoder we finally got around to prioritizing, and decided to go with a simpler user experience in v3. Check out this PR.