#19601 | Process fields of POJOs recursively

[DatApplePy]

Fixes #19601 (and fixes #19557) | [REQ][Java][Spring] Process and generate variables/objects (and their annotations) recursively in mustache

• Changes in mustache templates related to POJO and bean validation
  • Generates field, parameter and return type recursively along with their annotations and wrappers
  • Separates the use of Optional and JsonNullable
  • Field gets @Valid if it's a model
  • Apply @NotNull only if the field is required and not nullable
  • New vendor tags to define custom validation messages
    • x-size-message
    • x-min-message
    • x-max-message
    • x-decimalMin-message
    • x-decimalMax-message
• Fix issues in test cases caused by these changes

@cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02) @Zomzog (2022/09) @martin-mfg (2023/08)

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 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  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.
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