Scaler support for ElasticSearch Service on Elastic Cloud

[rcbop]

Proposal

Support connection with ElasticSearch on Elastic Cloud by adding two new parameters cloudID and apiKey to be used in the go-elasticsearch client initialization:

cfg := elasticsearch.Config{
  CloudID: "my-cluster:dXMtZWFzdC0xLZC5pbyRjZWM2ZjI2MWE3NGJm...",
  APIKey:  "VnVhQ2ZHY0JDZGJrUW...",
}

• These new parameters should be passed via Metadata or Auth sections.
• The scaler should exit with error if an user attempt to provide both endpoint addresses and cloudID at the same time.

Use-Case

Enable usage of ElasticSearch Service on Elastic Cloud by using only cloudID and apiKey

Anything else?

This is the first step to build a new custom scaler.