Allow the ACA environment's volume storage account to be customer-supplied

### 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.

I am trying to consolidate the auto generated "vol..." storage account for data volumes with a storage account created by "AddAzureStorage" that is used for blob storage. The reason for this is cost driven - currently this doubles our Microsoft Defender for Storage costs. Ideally we would like one storage account in the resource group for both application blobs and persistence of data volumes.

### Describe the solution you'd like

- Either an option on the the ACA environment to attach volume storage - similar to `WithHostStorage` for functions (#8143): `WithVolumeStorage(IResourceBuilder)`
- Or expose the the auto created ACA volume account name as a public `BicepOutputReference` so we can use `PublishAsExisting` on the storage account.

### Additional context

_No response_

Start by locating the ACA environment volume-storage implementation and the existing AddAzureStorage and WithHostStorage APIs. Compare the two proposed integration paths, then trace the related resource-generation behavior and tests; done means the environment can use a customer-supplied storage account without creating a separate volume account, or exposes its generated account name for reuse.

Written by the indexing model from the issue text.