Export also fieldCaps at the service

[gilkahan]

I'm submitting a...

[ ] Regression 
[ ] Bug report
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

fieldCaps is not exported

Expected behavior

Need to export fieldCaps also, or even to change the esClient at ElasticsearchService to be a protected so it will be possible to extend the service.

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Environment

Nest version: X.Y.Z

 
For Tooling issues:
- Node version: XX  
- Platform:  

Others:

[kamilmysliwiec]

ElasticsearchService now extends the native Client class.