Support getting topic names and message types

[nkoenig]

🎉 New feature

Summary

Support returning a list of topics and their message types using the websocket server.

Test it

Run the websocket server, and request "topics-types". You can test using https://gitlab.com/ignitionrobotics/web/app/-/merge_requests/111

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

[codecov]

Codecov Report

Merging #102 (270ee84) into ign-launch3 (77f736a) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-launch3     #102   +/-   ##
============================================
  Coverage        30.24%   30.24%           
============================================
  Files                3        3           
  Lines              734      734           
============================================
  Hits               222      222           
  Misses             512      512           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 77f736a...270ee84. Read the comment docs.

[german-e-mas]

LGTM! Works as intended.

[nkoenig]

@osrf-jenkins run tests please.