[FEATURE] API key auth support

[merlinz01]

API keys were added to OpenSearch 3.7. To use them, currently one must do this:

cfg := opensearch.Config{
    Header: make(http.Header),
}
cfg.Header.Set("Authorization", "ApiKey "+apikey)

It would be great if opensearch.Config would grow an APIKey field to make this seamless. I would be willing to attempt a PR for this.

[Ashwinnbr007]

@merlinz01 Can I work on this issue?

[merlinz01]

@Ashwinnbr007 Sure, at this point I don't have the time to do it myself.

[sean-]

It's just code, so "first to file" or "first-come, first-serve" is my motto. Life happens and we should have zero ego about the bits published so long as it gets done and is a net-positive to the code base (i.e. I have a zero-territorialness policy about development as long as it gets done and is good enough. "Perfect is the enemy of good.").