[feature] Support list vnets from subscription as a prompt

[vhvb1989]

Make azd to offer a prompt for selecting a vnet by listing all available vnets from a subscription.

Use same pattern as listing resource groups during resource group deployment.

Add also a meta-value for bicep-param for vnet, similar to how the location meta-value is supported.

See context: #3120 (reply in thread)

[ellismg]

I very much like this idea. It might be nice to expand this to support something like:

@metadata({azd: { type: 'resource', selector: '....' }}) where selector is some expression that allows folks to filter resources based on given types or other expressions?

It would be nice to leverage this in some unified way long term to support things like: "pick an existing database in your account to pass to this optional bicep parameter, or leave it blank so the infrastructure knows it needs to provision one".

[shaneholder]

It would be especially good if this applied to terraform templates as well, possibly using a commenting syntax.