RestClient should allow compressing request bodies

[swallez]

The Rest client has transparent support for compressed responses, but no support for compressed requests.

The low-level client should make it easy to compress request bodies, either globally or per-request. The high-level client should automatically benefit from this feature.

Compressing the request body should be driven by adding the corresponding header on RequestOptions, e.g.:

RequestOptions gzipOptions = RequestOptions.DEFAULT.toBuilder().addHeader("Content-Encoding", "gzip").build();
esClient.search(new SearchRequest("foo"), gzipOptions);

We can also add a more convenient RequestOption.compress(true) but ultimately that will set the header option as illustrated above, since that header can always be set directly.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Java Low Level REST Client)

[cykl]

This would be a great improvement. I spent a few hours today trying to compress request bodies but failed to find a user-level workaround.

Any user who pay outgoing data transfer should be greatly interested by this feature 💵