[BUG] Schema mappings in configuration file do not work

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

I'm getting an error when I try to generate a model with a schema mappings defined in a config file.

openapi-generator version

6.4.0

OpenAPI declaration file content or url

Generation Details

Generating with --schema-mappings argument on the command line works:
openapi-generator-cli generate -i common_components.yaml -g cpp-pistache-server --schema-mapping Arm=org.openapitools.server.model.Arm

Steps to reproduce

If I try to move the schema mapping to a config file, I get an error:

openapi-generator-cli generate -i common_components.yaml -g cpp-pistache-server --config config.yaml

Config.yaml looks like this:

schemaMappings: "Arm=org.openapitools.server.model.Arm"

Results in the following error:

[main] ERROR o.o.c.config.CodegenConfigurator - Cannot construct instance of `java.util.LinkedHashMap` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('Arm=org.openapitools.server.model.Arm')
 at [Source: (File); line: 2, column: 1] (through reference chain: org.openapitools.codegen.config.GeneratorSettings["schemaMappings"])
[main] ERROR o.o.c.config.CodegenConfigurator - Cannot construct instance of `java.util.LinkedHashMap` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('Arm=org.openapitools.server.model.Arm')
 at [Source: (File); line: 2, column: 1] (through reference chain: org.openapitools.codegen.config.GeneratorSettings["schemaMappings"])
Exception in thread "main" java.lang.RuntimeException: Unable to deserialize config file: config.yaml
	at org.openapitools.codegen.config.CodegenConfigurator.readDynamicSettings(CodegenConfigurator.java:171)
	at org.openapitools.codegen.config.CodegenConfigurator.fromFile(CodegenConfigurator.java:90)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:314)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

Related issues/PRs

Suggest a fix

Start with CodegenConfigurator.readDynamicSettings and fromFile, then inspect GeneratorSettings.schemaMappings while reproducing the command using config.yaml. Compare the config-file value with the working --schema-mapping argument; done means the schema mapping is accepted from config.yaml and generation completes without the deserialization error.