Add Key Vault approach for client secret

[guardrex]

Fixes #33933

Notes

• 🎉 Seems to work well, including during local development in VS.
• I am adding a bit on the initialization step for the Secret Manager tool. You asked about that before, and I responded that the user secret Id is already there. However, they really should get a unique Id, so I'm adding a bit here to have them run the init command ... and I've already pulled the <UserSecretId> from the sample app on Drop user secrets Id blazor-samples#379. I've already done the same thing for the OIDC-based samples and article on Drop hardcoded user secrets IDs blazor-samples#380 and Initialize the Secret Manger tool #33949.
• Check my naming of API! You know how badly a green dinosaur 🦖 can name things 🙈😆.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/security/blazor-web-app-with-entra.md	aspnetcore/blazor/security/blazor-web-app-with-entra
aspnetcore/grpc/why-migrate-wcf-to-dotnet-grpc.md	aspnetcore/grpc/why-migrate-wcf-to-dotnet-grpc