Introduce strict deprecation mode for low-level REST client

[jasontedor]

We want to surface deprecation warnings on the client side more visibly, as today they come back as headers which might not be being examined by an application developer using the Java low-level REST client. This issue proposes that we add a strict deprecation mode to the low-level REST client with the following properties:

• if strict mode is off (default for 6.x), we log all deprecation warning headers
• if strict mode is on, a deprecation warning header in the response fails the request
• we can consider in the future changing the default to strict mode being enabled

[elasticmachine]

Pinging @elastic/es-core-infra

[nik9000]

+1

I'd like to be able to use this strict mode in the high level rest client's tests as well.

[javanna]

Note that we intended to log all warnings whenever log level is set to WARN, but I am afraid that does not work anymore as the format of warnings responses header has changed. Would be nice to also expose warning headers as well as other headers (out of the scope of this issue) to the high-level REST client too, it should be possible now that we are duplicating response objects.

[javanna]

I was wrong above, warnings still get logged. Not sure if anything has to change on that end.