Allow overriding date for CEF Inputs

## What?

Include the "Allow overriding date?" option present in Syslog Inputs in CEF Inputs as well.

ref:

## Why?

By [customer request](https://app.hubspot.com/contacts/8267556/record/0-5/2050552112).

Besides, it does make sense that this should be an option for any Input that contains automatic timestamp parsers, like CEF, GELF, and Beats for example. Don't let upstream the poor timestamp formatting practices of certain vendors (_you know who you are_) degrade our users' experience with our awesome product!

## Your Environment

* Graylog Version: 5.1.7