[BUG] Options are not parsed correctly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
  • 4.1.2-SNAPSHOT
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

From #japanese slack channel:
https://openapi-generator.slack.com/archives/CLTHWFBEJ/p1566966541003500

--type-mappings object=Dict[str, Any],string=String

The parameter above is not parsed correctly.

Expect:

• object=Dict[str, Any]
• string=String

Actual:

• object=Dict[str
• Any]
• string=String

It's due to the splitCommaSeparatedList implementation that just split by comma:

https://github.com/OpenAPITools/openapi-generator/blob/046db19a8590f3557ae1e04be75792bbeb39cb69/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/OptionUtils.java#L54-L57

openapi-generator version

4.1.2-SNAPSHOT

Command line used for generation

openapi-generator \
generate \
 --input-spec openapi.yaml \
 --generator-name python \
 --output /local/out \
 --type-mappings object=Dict[str, Any]

Suggest a fix

• Improve the parsing implementation to respect symbols ([], <>)
• or use parsing library that satisfy the requirement

Start in modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/OptionUtils.java at the linked splitCommaSeparatedList implementation. Reproduce the issue with the documented openapi-generator command and type mapping, then verify that values containing brackets or angle brackets remain intact while separate mappings are still parsed separately.