[req] Allow models and apis list properties to span multi-lines

[btpnlsl]

fix Issue 19628

For consideration, this code change to DefaultGenerator.java allows API, MODELS, SUPPORTED_FILES & WEBHOOKS properties to include white space and newlines which are trimmed after splitting. This allows users to specify longer lists of models using maven without having to worry about extra spaces or newlines.

I opted to change DefaultGenerator instead of modifying the CodeGenMojo in the maven plugin since MODELS or APIS can be specified in either in modelsToGenerate or in the globalProperties/models setting. Doing so in DefaultGenerator allows for either method to span multi-lines. See the maven plugin README for an example of specifying MODELS via globalProperties or modelsToGenerate.

I've added a unit test in DefaultGeneratorTest that should cover all the properties except for WEBHOOKS. If there are some modifications I can make to the unit test to support multiple WEBHOOKS in the OpenAPI spec, let me know and I can add them.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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[wing328]

when you've time, can you please rebase this change with the latest master?

thank you

[btpnlsl]

Rebased and fixed a couple of variable names in the unit test.

[wing328]

thanks for the contribution.

let's give it a try