Add scalafmt plugin to scala-pekko client generator

[YB0y]

Adds scalafmt (Scalafmt 3.10.6, Scala 2.13 dialect) and sbt-scalafmt plugin (2.5.6) to the scala-pekko client generator, following the same pattern as #23273 (scala-sttp4).

Part of #23274

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Summary by cubic

Add scalafmt support to the Scala Pekko client generator to enforce consistent formatting in generated clients. Uses Scalafmt 3.10.6 with the Scala 2.13 dialect and the sbt-scalafmt plugin 2.5.6. Part of #23274 to standardize formatting across Scala generators.

• New Features
  • Generate .scalafmt.conf with the shared config (version 3.10.6, Scala 2.13).
  • Add project/plugins.sbt with sbt-scalafmt 2.5.6.
  • Wire templates in ScalaPekkoClientCodegen so new clients include both files.
  • Update Scala Pekko petstore sample to reflect the new files.

^{Written for commit ea44380. Summary will update on new commits.}

[YB0y]

Hi @wing328 Could you please review this PR and if it looks fine, could you merge this?

[cubic-dev-ai]

No issues found across 6 files

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[YB0y]

@wing328 Thank you for merging.
Shall I open more PRs for others?