e2e: extend FRR.Route struct to include AS-Path prepend information

Is your feature request related to a problem?

Currently, frrcontainer.FRR parses BGP route information into the FRR.Route struct, but it doesn't include parsed AS-Path prepend counts.

Describe the solution you'd like

We should extend the FRR.Route to natively parse and expose AS-Path prepends so downstream tests in frr-k8s and metallb don't need custom helper functions like AsPathPrependForPrefix.

Additional context

No response

I've read and agree with the following

• I've checked all open and closed issues and my request is not there.
• I've checked all open and closed pull requests and my request is not there.