System hooks API isn't exposed

[slonopotamus]

System Hooks API documentation. Seems like it is currently (as of 4.7.8) totally inaccessible via gitlab4j-api.

[gmessner]

This sub-API has not yet been added. It would be helpful if you could implement it and put in a PR, if you are not able to add it yourself, I can get to it in the next week or so.

[gmessner]

@slonopotamus
I've added initial support for the System Hooks API (SystemHooksApi).

I've also added a package that will process incoming system hook events, only have a couple of the event types implemented, but wanted to get your feedback before adding the remaining events. Checkout org.gitlab4j.api.systemhooks for this functionality.

[gmessner]

@slonopotamus
I released the initial functionality along with a bug fix for a problem with issue time estimates (Should have created a branch for this new feature).

I will be completing support for the other System Hook events in the next day or so.

[gmessner]

The System Hooks implementation has been completed and is released in version 4.7.14.