Resolve #1091 Require default feature for ICMP echo request auto reply

[aaronddavies]

This PR changes the auto reply for an ICMP echo request message (ping) to require a feature at compile time. The new feature is added to the default set, so no behavior change will occur for the default case.

To disable the auto reply, the user must either compile without default features or remove the feature from the default set.

The README is also updated.

Resolves #1091.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.43%. Comparing base (39cd44e) to head (40809ff).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1106      +/-   ##
==========================================
- Coverage   80.45%   80.43%   -0.03%     
==========================================
  Files          81       81              
  Lines       24461    24470       +9     
==========================================
+ Hits        19681    19682       +1     
- Misses       4780     4788       +8     

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