fix: Reduce the number of RPC calls when fetching attestations

[alainncls]

What does this PR do?

When getting one or multiple attestation(s), the calls to fetch the Schema are now made to the subgraph instead of the chain. This reduces the number of RPC calls to 0 for such operations.

Related ticket

Fixes #579

Type of change

• Chore
• Bug fix
• New feature
• Documentation update

Check list

• Unit tests for any smart contract change
• Contracts and functions are documented

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.63%. Comparing base (f607329) to head (1fd7b4b).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #580   +/-   ##
=======================================
  Coverage   93.63%   93.63%           
=======================================
  Files          20       20           
  Lines         330      330           
  Branches       76       76           
=======================================
  Hits          309      309           
  Misses         10       10           
  Partials       11       11           

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