Add option to build embedd broker as shared library

[NorbertHeusser]

Thank you for contributing your time to the Mosquitto project!

Before you go any further, please note that we cannot accept contributions if
you haven't signed the Eclipse Contributor Agreement.
If you aren't able to do that, or just don't want to, please describe your bug
fix/feature change in an issue. For simple bug fixes it is can be just as easy
for us to be told about the problem and then go fix it directly.

Then please check the following list of things we ask for in your pull request:

• Have you signed the Eclipse Contributor Agreement, using the same email address as you used in your commits?
• Do each of your commits have a "Signed-off-by" line, with the correct email address? Use "git commit -s" to generate this line for you.
• If you are contributing a new feature, is your work based off the develop branch?
• If you are contributing a bugfix, is your work based off the fixes branch?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run make test with your changes locally?

---

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.62%. Comparing base (3c1a8c1) to head (9886279).
Report is 23 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3057      +/-   ##
===========================================
+ Coverage    81.35%   81.62%   +0.26%     
===========================================
  Files          191      196       +5     
  Lines        31780    32163     +383     
===========================================
+ Hits         25855    26253     +398     
+ Misses        5925     5910      -15     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.