[csharp][generichost] Add OnCreated to JsonConverter

[devhl-labs]

Adding OnCreated to the JsonConverter so we have an appropriate place to set the date format.

PR checklist

• Read the contribution guidelines.
• 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.
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[wing328]

users may consider this (static to non-static) a breaking change.

we may need to provide a way to fallback if users ask for it.