feat: add write-only secrets attributes for webhook resources

[tdyas]

Resolves #3628

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Before the change?

• The github_repository_webhook and github_organization_webhook resources currenrly only support a secret attribute to set the webhook's secret. This secret is persisted in Terraform state which is suboptimal from a security perspective.

After the change?

• In addition to a secret attribute, github_repository_webhook and github_organization_webhook now support a write-only secret_wo attribute (and related secret_wo_version attribute for triggering updates) as per conventions used in other Terraform providers. Terraform's ephemeral resources support can now be used together with the write-only secret_wo attribute to ensure webhook secrets are never persisted in Terraform state.

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

No. The new secret_wo and secret_wo_version fields were added and the secret field remains in place and available for use by users. The two schemes are marked as conflicting in the schema so Terraform and logic in this PR should prevent users from trying to use both schemes.

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

👋 Hi, and thank you for this contribution!

This repo is maintained by GitHub and community members on a best-effort basis. We'll get to this as soon as we can.

You can help us prioritize by joining the discussion on open issues and PRs, sharing details on the changes you need, and reviewing other contributions.

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