Upgrade OSBAPI support to 2.16

Issue

I'm trying to understand how far away from 2.16 compatibility we actually are.

I've taken this list from the OSBAPI 2.16 spec

I'm not really sure where we are on any of these points, so I'll update this list as I find out. Some of these are optional to achieve compatibility.

• ❓Add guidance of how long the state of an operation SHOULD be remembered
• ❓Add guidance to handle 500 errors from Service Instance update
• ❓Add guidance to handle equests with invalid data
• ❓Allow Service Brokers to indicate if a Service Instance is still usable after a failed update or deprovisioning and if an update can be repeated
• ❓Specify that Platforms SHOULD NOT reuse IDs
• ❓Allow Service Brokers to return additional information on GET requests
• ❓Add CF and K8s annotations to the profile document
• ❓Add support for ETag and If-Modified-Since headers
• ❓Clarify the response code when Platform does not provide the REQUIRED API version header
• 🚀 Service Instances can be labelled with information defined by the Service Broker
  • Implemented by #4699 (as well as a 2.17 extension)

Legend:

• ❓status unknown
• ✅ done
• 🚫 not done
• 🚀 in progress

Start by comparing the listed items with the OSBAPI 2.16 specification's changes-since-v2.15 section and reviewing issue #4699. Done means every compatibility point has an evidence-based status and the remaining required work is clearly scoped.