#23273 Add scalafmt (plugin) to scala-sttp4 client

cc @clasnake (2017/07), @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04) @fish86 (2023/06)

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)
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Summary by cubic

Adds Scalafmt to the Scala STTP v4 client generator to ensure consistent formatting. The generator now outputs a .scalafmt.conf and enables the sbt-scalafmt plugin.

• New Features

  • Generates .scalafmt.conf (Scalafmt 3.10.6, Scala 2.13 dialect).
  • Adds project/plugins.sbt with org.scalameta sbt-scalafmt 2.5.6.
  • Updates Scala STTP v4 samples to include these files.

• Refactors

  • Stops generating DateSerializers.scala.