[csharp-netcore] Adds features for #11010 and #11001

[RubberChickenParadise]

Adds the features requested in #11010 and #11001 for csharp-netcore client generation

PR checklist

• [ x] Read the contribution guidelines.
• [ x] 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.
• [x ] Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  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.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• [x ] File the PR against the correct branch: master (5.3.0), 6.0.x
• [x ] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

[wing328]

@RubberChickenParadise can you please resolve the merge conflicts when you've time?

If you're using Windows, pleas use Git Bash to run the following (step 3):

./mvnw clean package 
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh

let us know if you need any help.

[RubberChickenParadise]

@wing328 i have resolved the conflicts once already.

I can do it again

[wing328]

@RubberChickenParadise it shouldn't use ''. Please give Git bash (windows) a try to see if this's still repeatable.

[RubberChickenParadise]

@wing328 Getting some build errors with ./mvnw clean package on git bash. not sure whats going on as I am not a java dev.

[INFO]
[INFO] Results:
[INFO]
[ERROR] Failures:
[ERROR] DefaultGeneratorTest.supportCustomTemplateEngine:311 ▒ Runtime Could not gener...
[ERROR] StringHelpersTest.startsWithSectionalTest:47->evaluate:28 ▒ Handlebars inline@...
[ERROR] StringHelpersTest.startsWithTest:61->evaluate:28 ▒ Handlebars inline@43bd77be:...
[ERROR] StringHelpersTest.startsWithYesOverrideTest:81->evaluate:28 ▒ Handlebars inlin...
[INFO]
[ERROR] Tests run: 1553, Failures: 4, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for openapi-generator-project 5.4.0-SNAPSHOT:
[INFO]
[INFO] openapi-generator-project .......................... SUCCESS [ 2.031 s]
[INFO] openapi-generator-core ............................. SUCCESS [ 9.671 s]
[INFO] openapi-generator (core library) ................... FAILURE [02:03 min]
[INFO] openapi-generator (executable) ..................... SKIPPED
[INFO] openapi-generator (maven-plugin) ................... SKIPPED
[INFO] openapi-generator-gradle-plugin (maven wrapper) .... SKIPPED
[INFO] openapi-generator-online ........................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 02:16 min
[INFO] Finished at: 2022-01-04T17:01:24-06:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:3.0.0-M5:test (default-test) on project openapi-generator: There are test failures.