internet-apps: (fixes #784) Merge V4Ping and Ping6 in a single application

This is the "main" MR for Chandrakant Jena's NSOC-22 project.

To summarise the project goals, its aim is to:

1. Merge V4Ping and Ping6 in a single application (Ping),
2. Align the behaviour with Linux's version of ping,
3. Deprecate the old applications and their helpers.

List of things to do:

• Cleanup the deprecation of classes
• Finish porting the existing code to the new Ping application
• Remove "hard" timeout of replies (Linux does not have it)
• Add duplicate replies handling
• Allow Multicast (byproduct of the two above)
• More tests (IPv6, selected packet loss)
• Documentation (task for @tomhenderson)

Further enhancements might be:

• Include more tests
• Handle ICMP errors, e.g., destination unreachable-- these are not generated by ns-3 routers, however