Define available subscriptions in config.yaml

[askew]

With the multi-subscription capability enabled by PR #4455, it would be nice to configure available subscriptions when the core is deployed.

PR #4429 also enables array items to be added to config.yaml so this means the list of available subscriptions could be added.

The core deployment could then grant the permissions required to the id-vmss-treid managed identity when it is deployed.

The list of available subscriptions could also be made available via the API which would enable a more friendly drop-down list selection when creating a workspace.

[SiobhanBaynes]

Currently Billing Reader needs to be added manually to the managed identity id-api-[TRE_NAME] to allow it to query the costs. We could also add this RBAC permission if we had the subscription ID at deploy time.