Configurable first sync depth

[gammazero]

There is now a dagsync.Subscriber option to configure the first sync depth for a new provider.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a71a4b6) 62.06% compared to head (fbf426c) 61.90%.

Files	Patch %	Lines
dagsync/option.go	57.14%	2 Missing and 1 partial ⚠️
dagsync/subscriber.go	80.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
- Coverage   62.06%   61.90%   -0.17%     
==========================================
  Files          49       49              
  Lines        4302     4318      +16     
==========================================
+ Hits         2670     2673       +3     
- Misses       1335     1347      +12     
- Partials      297      298       +1     

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