Suggest REST endpoint fix for POST/PUT

[aleph-zero]

Fix an odd behavior whereby a user can create documents with id=_suggest
by issuing POST/PUT requests to
http://localhost:9200/{index}/{type}/_suggest.

This fix intercepts such requests and returns a 400 BAD_REQUEST
response.

Additionally, this commit includes an extension to the standard
integration test facilities which allows a subclass to use a RestClient
instance to issue REST requests.

Closes #5442

[javanna]

I'm on the fence on this change... isn't the result the same as preventing a document to be indexed if it has _suggest as id? Couldn't we handle this in the index api then instead of adding support for PUT here to be able to throw an error?

Also, I'd much rather prefer to keep testing the REST layer using yaml tests, which are shared between all the clients as well. Handling this in the index api would make it easier to test it as well.

Maybe we'd need to consider validating the document ids and prevent users from using any of our own registered _endpoints just to avoid confusion?

[s1monw]

+1 to validiate doc IDs based on the endpoints we have so it's actually dynamic!

[clintongormley]

Just because an ID doesn't clash with an existing endpoint doesn't mean that it won't clash in the future. I think we should say that IDs can't begin with an underscore.

Closing in favour of #6736