Having announce.Receiver is optional for Subscriber

[gammazero]

It is not always necessary to have an announcement receiver driving the Subscriber, such as when it is used within a CLI to get advertisements on-demand only. Making it optional avoids the setup time and possible unwanted automated behaviors that result from having the announcement receiver enabled.

[codecov-commenter]

Codecov Report

Patch coverage: 61.70% and project coverage change: -0.48% ⚠️

Comparison is base (2f35326) 53.65% compared to head (bc1be6e) 53.18%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   53.65%   53.18%   -0.48%     
==========================================
  Files          65       65              
  Lines        5155     5148       -7     
==========================================
- Hits         2766     2738      -28     
- Misses       2097     2117      +20     
- Partials      292      293       +1     

Files Changed	Coverage Δ
announce/option.go	70.37% <ø> (-18.52%)	⬇️
dagsync/option.go	43.15% <37.50%> (-2.30%)	⬇️
announce/receiver.go	70.18% <63.63%> (-2.76%)	⬇️
dagsync/subscriber.go	76.45% <67.85%> (-0.70%)	⬇️

... and 2 files with indirect coverage changes

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