Deploy the groundstation into an existing Azure Virtual Network

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

Today, the Bicep template deploys a new virtual network, subnet, and network security group.

**Describe the solution you'd like**

If I have an existing network, I'd like to reuse it by providing it as parameters to the template.

**Describe alternatives you've considered**

There's a version of this working at `glenn/useExistingNetwork` by use of a `useExistingNetwork` parameter: https://github.com/Azure/mock-spacestation/blob/glenn/useExistingNetwork/AzureVM.bicep

**Additional context**

n/a