[CSHARP] Add cache to efficiently lookup CodegenModel in csharp codegen to make csharp generator faster

[ivanleb]

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.
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

The csharp generator works unacceptably slow (~ few hours) when it consumes a large enough spec file.
F.e. :
vi_json_openapi_specification_v8_0_3_0.json

Meanwhile any other generator works in a minute with the same spec.

To reproduce this behavior run
java -jar modules\openapi-generator-cli\target\openapi-generator-cli.jar generate -i vi_json_openapi_specification_v8_0_3_0.json -g csharp --skip-validate-spec -o <your output directory>

To compare to java generator run
java -jar modules\openapi-generator-cli\target\openapi-generator-cli.jar generate -i vi_json_openapi_specification_v8_0_3_0.json -g java --skip-validate-spec -o <your output directory>

The fix uses memoization and continues the fix from 10152

The generated code is the same as before.

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/18374180139/job/52465992889?pr=22094

can you please merge the latest master into your branch? I believe this issue no longer happens with the latest master