[BUG] Options are not parsed correctly

Created by: ackintosh

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