Add support for include-file-directive in config files.

[t2gran]

Summary

This feature make it possible to inject the content of another file into a configuration file. It make it possible to keep part of the configuration in separate files. To include the content of a file use ${includeFile:FILE_NAME}. The FILE_NAME must be the name of a file in the configuration directory. Relative paths are not supported.

To allow both files (the configuration file and the injected file) to be valid JSON files a special case is supported; If the include file directive is quoted, then the quotes are removed if the text inserted is valid JSON. We check that the text to inject start with a `{` and ends with `}`.

Variable substitution is performed on the configuration file after the include file directive; Hence
variable substitution is also performed on the text in the injected file, but not inside the ${includeFile:FILE_NAME}.

Here is an example including variable substitution, assuming version 2.1.0 of OTP:

// build-config.json
{
  "storage" : "${includeFile:storage.json}"
} 

// storage.json
{
  "streetGraph": "street-graph-v${maven.version}.obj"
}

The result will look like this:

{
  "storage" : {
    "streetGraph": "street-graph-v2.1.0.obj"
  }
} 

Issue

There is no issue for this, but this feature where requested as part of the #3760.

Unit tests

✅

Code style

✅

Documentation

✅

[t2gran]

In the dev-meeting today, I got some feedback that the "preview version" of this did not worh, due to white-space before/after the {/}. I have updated the unit test to cover this case and fixed it.

[leonardehrenfried]

I ran the code and it does what it should. The code also looks good.