expose ES createClient to plugins

[mshustov]

Summary

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Checklist

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• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [ ] Documentation was added for features that require explanation or tutorials
• [] Unit or functional tests were updated or added to match the most common scenarios
• [ ] This was checked for keyboard-only and screenreader accessibility

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• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

Dev docs

New platform plugins may create their custom elasticsearch clients with elasticsearch.createClient.

class MyPlugin {
 public async setup(core: CoreSetup) {
   this.clusterClient = core.elasticsearch.createClient('name', configObject);
   const result = await this.clusterClient.callAsInternalUser('/endpoint');

[elasticmachine]

Pinging @elastic/kibana-platform

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[joshdover]

Nice and easy

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request