Allow overriding RPC

[0xFirekeeper]

TOOL-2920

Pass optional rpcOverrides mapping to client

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PR-Codex overview

This PR introduces enhancements to the ThirdwebClient and ThirdwebRPC classes, adding support for custom RPC URLs via a Dictionary<BigInteger, string> RpcOverrides. It also includes tests for verifying the functionality of these overrides.

Detailed summary

• Added internal Dictionary<BigInteger, string> RpcOverrides to ThirdwebClient.
• Updated ThirdwebClient constructor and Create method to accept rpcOverrides.
• Introduced internal Uri RpcUrl in ThirdwebRPC.
• Modified RPC instance retrieval to use custom RPC from RpcOverrides.
• Added unit tests for various scenarios involving RPC overrides.
• Removed commented-out test for caching behavior.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.72%. Comparing base (1ff3a70) to head (6b87a56).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   76.64%   76.72%   +0.07%     
==========================================
  Files          28       28              
  Lines        4788     4799      +11     
  Branches      662      665       +3     
==========================================
+ Hits         3670     3682      +12     
  Misses        945      945              
+ Partials      173      172       -1     

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