Support for Elastic Cloud ID

[philkra]

This PR adds support for:

• Elastic Cloud ID
• APIKey Authentication

When connecting to Elastic Cloud, the best practices should be used such as e.g. http compression.

[ezimuel]

I would like to propose the following changes in order to simplify the user experience:

• since the base64 value of id:api_key is not generated by Elastic Cloud (UI or throught API), I don't think we need to offer ClientBuilder::setApiKeyAuthentication() function;

• removing the setApiKeyAuthentication method, we can simplify the naming using ClientBuilder::setApiKey($id, $key) instead of setApiKeyPairAuthentication($id, $key);

• when we use the ApiKey we should remove the Basic Authentication in the HTTP request;

• the usage of the helper class ElasticCloudIdParser just to extract the cluster name and DNS it seems to be over-engineering. I think we can manage it directly into ClientBuilder.

• use of ClientBuilder::setElasticCloud(string $cloudId, string $username, string $password) instead of setElasticCloudId(string $cloudId, ?string $username = null, ?string $password = null); since we are using Elastic Cloud service we are building the hosts array explicit and these parameters cannot be optional.

Moreover, I just discovered that Elasticsearch supports also OAuth2 token, I think we should support also this feature. We can discuss it later if you want.