'include functionality' for logstash config files

We are currently using a lot of different logstash config files for several sources, and keeping tracking of the configs in a modular manner is not easily possible. Are there any plans on adding some sort of 'include functionality' to logstash.

For example:

A.conf :

```
filter {
.... # do stuff A
}
```

B.conf :

```
filter {
.... # do stuff B
}
```

In the actual config file We would use something like:

run.conf :

```
input { ... }

include('B')
include('C')

output { ... }
```

---

We are currently using symlinks and whole directories as config source as a workaround, but running into different problems here.

Start by tracing Logstash's configuration parsing and config-source handling; the issue does not name a file, test, or entry point. Compare the requested include('B') and include('C') behavior with the current symlink and directory workaround. Done should define supported include syntax, composition semantics, and coverage for modular configuration files.

Written by the indexing model from the issue text.