Support AzureWebJobsSecretStorageType=None, NullSecretsRepository

#8083 / [v4 PR] provides a simple fix for #5922, but the approach is somewhat fragile. A more robust approach would be to implement a NullSecretsRepository, while maintaining the current behavior of defaulting to blob storage.

| `AzureWebJobsSecretStorageType` | `AzureWebJobsStorage` | Secret Provider |
|---------------------------------------|----------------------------|------------------|
| `null` | `null` | NullSecretsRepository |
| `null` | exists | BlobStorageSecretsRepository |
| `None` | `null` | NullSecretsRepository |
| `None` | exists | NullSecretsRepository|

In the case where NullSecretsRepository is used, all authentication will be done through JWT token / bearer auth.

Care should be taken to ensure that admin endpoints cannot be successfully called with blank keys, ex. `/admin/host/status?code=`

Start by tracing how AzureWebJobsSecretStorageType and AzureWebJobsStorage select the secret provider, then inspect the authentication path for admin endpoints. Done means None selects NullSecretsRepository in every table case, null storage preserves blob-storage defaults where specified, bearer authentication works, and blank admin keys cannot authorize requests.

Written by the indexing model from the issue text.