X-Opaque-Id support

[delvedor]

This pr adds the support for a easy way to set the X-Opaque-Id header.

The X-Opaque-Id should be configured in each request, for doing that you can use the opaqueId option, as you can see in the following example.
The resulting header will be { 'X-Opaque-Id': 'my-search' }.

const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  node: 'http://localhost:9200'
})

client.search({
  index: 'my-index',
  body: { foo: 'bar' }
}, {
  opaqueId: 'my-search'
}, (err, result) => {
  if (err) console.log(err)
})

Sometimes it may be useful to prefix all the X-Opaque-Id headers with a specific string, in case you need to identify a specific client or server. For doing this, the client offers a top-level configuration option: opaqueIdPrefix.
In the following example, the resulting header will be { 'X-Opaque-Id': 'proxy-client::my-search' }.

const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  node: 'http://localhost:9200',
  opaqueIdPrefix: 'proxy-client::'
})

client.search({
  index: 'my-index',
  body: { foo: 'bar' }
}, {
  opaqueId: 'my-search'
}, (err, result) => {
  if (err) console.log(err)
})

[stale]

We understand that this might be important for you, but this issue has been automatically marked as stale because it has not had recent activity either from our end or yours.
It will be closed if no further activity occurs, please write a comment if you would like to keep this going.

Note: in the past months we have built a new client, that has just landed in master. If you want to open an issue or a pr for the legacy client, you should do that in https://github.com/elastic/elasticsearch-js-legacy