feat: make return value not optional in provider API functions

[federicobond]

This PR

Modifies the get_provider and get_provider_metadata return types to guarantee that they don't return a None value.

The default provider is initialized as a NoOpProvider at module loading time, so this simplifies the usage of the provider functions for clients.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1282bab) 94.02% compared to head (8329e89) 94.41%.

❗ Current head 8329e89 differs from pull request most recent head c76bc8a. Consider uploading reports for the commit c76bc8a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #270      +/-   ##
==========================================
+ Coverage   94.02%   94.41%   +0.39%     
==========================================
  Files          17       17              
  Lines         452      448       -4     
==========================================
- Hits          425      423       -2     
+ Misses         27       25       -2     

Flag	Coverage Δ
unittests	94.41% <100.00%> (+0.39%)	⬆️

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