Remove Hardcoded XML Configuration Generation

[Anmol202005]

Ref: #76 (comment)

Currently generating Checkstyle configuration XML manually using hardcoded strings:

public static void writeConfigurationToXml(CheckConfiguration config, String filePath)
        throws IOException {
    try (BufferedWriter writer = new BufferedWriter(new FileWriter(filePath))) {
        writer.write("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n");
        writer.write("<!DOCTYPE module PUBLIC \"-//Checkstyle//DTD Checkstyle "
                + "Configuration 1.3//EN\" \"configuration_1_3.dtd\">\n");
        writeModule(config, writer);
    }
}

Issue:

Header input files cannot contain config comments since they expect license headers as the first comment, making config extraction impossible.

Goal:

Investigate alternatives to remove hardcoded XML generation and find a cleaner approach for configuration handling.

[Anmol202005]

@rdiachenko
Checkstyle’s ConfigurationLoader has hardcoded DTD validation. Without a DOCTYPE, the parser expects root=null but encounters .
Since DOCTYPE is mandatory, there’s no way to disable validation in Checkstyle.

Even if the HeaderCheck restriction was absent and we could directly load the config from input, we would still need to include the DOCTYPE.

[rdiachenko]

ok, I need to think on this one, it is not critical, let's postpone it for now