Unable to provide custom files for generation [BUG]

🐛 Bug Report:

Describe the bug

The openapi generator specifies the ability to provide custom files/templates (https://openapi-generator.tech/docs/customization).
This feature does not work with the current version of the CLI.

Steps to Reproduce

Steps to reproduce the behavior:

1. Install latest version of openapi-generator-cli (currently 5.4.0) npm install openapi-generator-cli@5.4.0

2. Provide the following for the openapitools.json file (note the files keyword), e.g.

   {
       "$schema": "/path/to/schema",
       "generator-cli": {
           "version": "5.4.0",
           "generators": {
                "nodejs": {
                    "templateDir": "#{cwd}/nodejs/templates",
                    "generator-name": "typescript",
                    "output": "#{cwd}/nodejs/sdk",
                    "inputSpec": "#{cwd}/openapi.json",
                    "additionalProperties": {
                       "esTargetVersion": "ES2020",
                       "moduleResolution": "nodejs",
                       "framework": "fetch-api",
                       "platform": "node",
                       "license": "MIT",
                       "useInversify": false,
                       "legacyDiscriminatorBehavior": false
                    },
                    "files": {
                        "api_test.mustache": {
                           "templateType": "ApiTests",
                           "destinationFilename": "spec/api_test.spec.ts"
                        }
                      }
                   }
                }
           }
       }
   }
   

3. Execute the generator
   openapi-generator-cli generate --generator-key nodejs

4. See error

[[nodejs] #{cwd}/openapi.json] [error] Found unexpected parameters: [--files=api_test.mustache=[object Object]]
[[nodejs] #{cwd}/openapi.json] 
[[nodejs] #{cwd}/openapi.json] See 'openapi-generator-cli help' for usage.

  java -jar "node_modules/@openapitools/openapi-generator-cli/versions/5.4.0.jar" generate
--input-spec="/openapi.json"
--template-dir="/openapi-spec/nodejs/templates"
--generator-name="typescript" 
--output="/nodejs/sdk" 
--additional-properties="esTargetVersion=ES2020,moduleResolution=nodejs,framework=fetch-api,platform=node,license=MIT,useInversify=false,legacyDiscriminatorBehavior=false" 
--files="api_test.mustache=[object Object]" exited with code 1

Expected behavior

the "files" keyword gets resolved properly and all sub-properties are provided to the cli.

Operation System (please complete the following information):

• OS: MacOS
• Version 12.6.1

Package System (please complete the following information):

• Version 5.4.0

Additional context

The typescript generator doesn't ship with a test runtime, so I was in the process of using the customization feature - ran into this instead.