[FEAT]: support write-only secrets attributes for webhook resources

### Describe the need

The webhook resources (`github_repository_webhook` and `github_organization_webhook`) have a `secret` attribute which is not write-only, and thus the secret is persisted in Terraform state.

A better pattern used in other Terraform providers is to support write-only attributes `secret_wo` and `secret_wo_version` so that the secrets can be sourced from ephemeral resources and not persisted to the Terraform state nor included in a plan.

### GitHub Installation Type

- [ ] GitHub.com (Free, Pro, or Team)
- [ ] GitHub Enterprise Server (on-premises)
- [ ] GitHub Enterprise Cloud with Personal Accounts (github.com)
- [ ] GitHub Enterprise Cloud with Managed Users/EMU (github.com)
- [ ] GitHub Enterprise Cloud with Data Residency (*.ghe.com)
- [ ] I don't know

### Relevant log output

```shell

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Locate the implementations and tests for the github_repository_webhook and github_organization_webhook resources, then review how other Terraform providers in this repository define write-only attributes. Done means both resources support secret_wo and secret_wo_version while keeping secrets out of Terraform state and plans.

Written by the indexing model from the issue text.