Include dhstore into e2e test

[ischasny]

• fix a bug in the double hashed client that caused an invalid memory access if extended providers are nil;
• add a new flag to storetheindex to specify dhstore url on initialisation;
• include dhstore and double hashed client into e2e test;
• allow specifying different urls for dhstore and storetheindex in the double hashed client so that the client can work without indexstar running in front.

Fixes #1214

[codecov-commenter]

Codecov Report

Base: 54.00% // Head: 54.56% // Increases project coverage by +0.56% 🎉

Coverage data is based on head (f60eadb) compared to base (0b84c6f).
Patch coverage: 59.09% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1248      +/-   ##
==========================================
+ Coverage   54.00%   54.56%   +0.56%     
==========================================
  Files         150      150              
  Lines       14958    14969      +11     
==========================================
+ Hits         8078     8168      +90     
+ Misses       5999     5899     -100     
- Partials      881      902      +21     

Impacted Files	Coverage Δ
command/flags.go	0.00% <ø> (ø)
command/init.go	48.51% <16.66%> (-2.02%)	⬇️
api/v0/finder/client/http/dhash_client.go	44.86% <75.00%> (+44.86%)	⬆️
dagsync/test/util.go	82.93% <0.00%> (+0.47%)	⬆️
dagsync/p2p/protocol/head/head.go	78.16% <0.00%> (+5.74%)	⬆️

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