[enhancement]: simplify DataSourceEc2.convert_ec2_metadata_network_config

[aciba90]

Enhancement

See https://github.com/canonical/cloud-init/pull/4799#pullrequestreview-1866723421

With this change, convert_ec2_metadata_network_config is getting a bit long and harder to follow. I think it'd be better if we could split the policy based routing pieces into its own function(s) and perhaps use a single implementation for the v4 and v6 pieces.

[aciba90]

Tracking in https://warthogs.atlassian.net/browse/SC-1717.