refactor(feeder): use the new includeSignature flag in the get_state_update feeder endpoint

[thiagodeev]

• Combine get_state_update + get_signature feeder calls into a single request by passing includeSignature=true to the get_state_update endpoint.
• Update clients/feeder.Client.StateUpdateWithBlock to accept an includeSignature flag and decode the signature directly from the response.
• Refactor adapters/sn2core.AdaptBlock to take []*felt.Felt instead of *starknet.Signature.
• Extend starknet.StateUpdateWithBlock with an optional Signature []*felt.Felt field, populated when includeSignature=true.
• Drop the now-redundant f.client.Signature(ctx, blockID) call in starknetdata/feeder.stateUpdateWithBlock.
• Reformat clients/feeder/testdata/**/state_update_with_block/*.json fixtures by adding the signature field to it. The signature values were taken from the signature folder

Reference: https://demerzelsolutions.slack.com/archives/C03090TS3TK/p1777375140952279

Note:

• this PR can only be merged after the feeder implements this new feature

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.88%. Comparing base (af1fe00) to head (b730843).

Files with missing lines	Patch %	Lines
starknetdata/feeder/feeder.go	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3601      +/-   ##
==========================================
+ Coverage   75.87%   75.88%   +0.01%     
==========================================
  Files         387      387              
  Lines       34918    34914       -4     
==========================================
+ Hits        26493    26494       +1     
+ Misses       6556     6553       -3     
+ Partials     1869     1867       -2     

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