#25315 [C#][generichost] Support skipping certificate validation for HttpClient

Description

Add certificate-validation configuration to the C# generichost client. The generated extensions support custom validation callbacks and SkipCertificateCheck() for both HttpClientHandler and SocketsHttpHandler.

Use an explicitly typed options callback and a direct certificate-validation delegate to maintain compatibility with netstandard2.0. Update the generichost samples and document that skipping validation is for development or testing only.

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.
  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.
• If your PR is targeting a particular programming language, @mention the @mandrean @shibayan @Blackclaws @lucamazzanti members, so they are more likely to review the pull request.

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Summary by cubic

Adds certificate-validation configuration to the C# generichost client, so generated HTTP clients can skip certificate checks or use custom validation callbacks.

Changes

• Introduces ConfigureRemoteCertificateValidationCallback for HttpClientHandler and SocketsHttpHandler.
• Uses an explicitly typed options callback to remain compatible with netstandard2.0.
• Updates generichost samples and READMEs, with a note that skipping validation is for development or testing only.