Add Fuzz-testing

### Is your feature request related to a problem? Please describe.
Fuzz testing or Fuzzing is a Black Box software testing technique, which basically consists in finding implementation bugs using malformed/semi-malformed data injection in an automated fashion.

### Describe the solution you'd like
Integrate the project with [OSS-Fuzz](https://github.com/google/oss-fuzz) by following the instructions [here](https://google.github.io/oss-fuzz/).

**From fluentd maintainer:**

Some input plugins receive data online, so it may be a good idea to add a fuzz test for them first.

- https://docs.fluentd.org/input/forward
- https://docs.fluentd.org/input/udp
- https://docs.fluentd.org/input/tcp
- https://docs.fluentd.org/input/http
- https://docs.fluentd.org/input/syslog

### Describe alternatives you've considered

N/A

### Additional context

Part of #4601

Start with the OSS-Fuzz instructions and the fluentd input plugin documentation for forward, UDP, TCP, HTTP, and syslog. Determine which online input plugins should receive fuzz tests first, then verify that the project is integrated with OSS-Fuzz and the selected inputs are covered.

Written by the indexing model from the issue text.