Show subscribers info when running topic info

[caguero]

🎉 New feature

Summary

This patch includes the subscribed topics in the output of gz topic -i.

Test it

In one terminal run:

gz topic -e -t /foo

In another terminal run:

gz topic -i -t /foo

Your output should be:

No publishers on topic [/foo]
Subscribers [Address, Message Type]:
  tcp://172.17.0.1:39133, google.protobuf.Message

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #384 (70e44a3) into main (4bbf528) will decrease coverage by 0.03%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
- Coverage   87.58%   87.55%   -0.03%     
==========================================
  Files          60       60              
  Lines        5236     5257      +21     
==========================================
+ Hits         4586     4603      +17     
- Misses        650      654       +4     

Impacted Files	Coverage Δ
src/Node.cc	91.76% <90.00%> (-0.55%)	⬇️
src/cmd/gz.cc	95.58% <100.00%> (+0.27%)	⬆️
include/gz/transport/Discovery.hh	84.64% <0.00%> (-0.11%)	⬇️

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