Introduce startClientFactories method to Servient

[JKRhb]

As it is the case with the Servient class in node-wot, the start method in dart_wot is currently marked as async. This makes sense in general, as the start methods of the servers registered with the Servient are also asynchronous.

However, as the corresponding init method of the ProtocolClientFactoriesis not asynchronous (at least in dart_wot), it would be possible to start the Servient synchronously and not require the surrounding environment to be async as well. Having such a synchronous, partial start method can therefore be beneficial for applications that only act as consumers.

To address this use case, this PR introduces a synchronous startClientFactories method that basically does what it advertises. It also applies some minor refactoring of the server starting code.

As, as far as I can tell, the ProtocolClientFactorys in node-wot do not actually perform any actions at the moment in their init methods, maybe this is also something that considered for node-wot as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.40%. Comparing base (d6acac1) to head (a8f519f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
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+ Coverage   63.34%   63.40%   +0.06%     
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  Files          78       78              
  Lines        2275     2279       +4     
==========================================
+ Hits         1441     1445       +4     
  Misses        834      834              

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[JKRhb]

I will go ahead and merge this PR, but I applied the "Discuss in Committer Meeting" label, so maybe we can have a brief technical discussion about this in one of our next calls.