Add support for OpenBao

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Add support for [OpenBao](https://openbao.org/), the Linux Foundation–maintained fork of HashiCorp Vault, to provide secure management of secrets, certificates, and keys.
This integration will also enable full offline and air‑gap deployments for network‑constrained environments, ensuring the secure retrieval, storage, and rotation of sensitive assets as expected in modern infrastructures.

### Describe the solution you'd like

Introduce a generic Vault-API abstraction within Aspire that allows seamless switching between Azure Key Vault and OpenBao based on environment constraints defined by Aspire users.
This would ensure applications can consistently retrieve/manage secrets, certificates, and keys, regardless of the underlying provider, whether Azure Key Vault, OpenBao, or any other supported option in the future.

### Additional context

There have been discussions about managing certificates injection from Vaults in the following PR : [Add support for configuring TLS termination for supported resources #12506](https://github.com/dotnet/aspire/pull/12506)