Peering config validation: Revise and check

[qmonnet]

We need to revise the validation steps for the peering, as per #1150.

This includes, but is not limited to:

• Peering config validation: Support overlapping prefixes within a VpcExpose #1279
• Peering config validation: Reject reserved prefixes, port 0 #1280

We also want to extend unit tests to make sure we reject all relevant cases discussed in #1150.