Adding option to invoke operations in jolokia2 agent

## Feature Request
Besides reading attributes the jolokia2 agent should be able to invoke operations from custom Mbeans through the JMX.

### Proposal:
We would like to see operations to be captured from invoked actions

### Current behavior:
Current behaviour supports only read requests and attributes ie:
curl http://localhost:10012/jolokia/read/java.lang:type=Memory | python -m json.tool

### Desired behavior:
I would like to have also operations supported for custom Mbeans(for our applications) created in Spring.boot. An example is:
http://localhost:10162/jolokia/exec/org.springframework.boot:type=Endpoint,name=Health/health/ | python -m json.tool
While I can see the result in command line I cannot capture it with telegraf

Start with the jolokia2 agent's existing handling of read requests and compare it with the shown /jolokia/exec/... request for a Spring Boot custom MBean. Verify how Telegraf captures the operation result, and consider the work complete when invoked operations are captured for custom MBeans as well as attributes.

Written by the indexing model from the issue text.