Boxes PubsubClientError's ConnectionError and WsError

[brooksprumo]

Problem

Issue #6278: The PubsubClientError triggers the result_large_err clippy lint. This is because two variants, ConnectionError and WsError, contain a tungstenite::Error, which is 136 bytes. To fix this, and since this is only needed in the error case, we can box the tungstenite::Error.

Similar to #6293.

Summary of Changes

Boxes PubsubClientError's ConnectionError and WsError, which both contain tungstenite::Error.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.8%. Comparing base (51c33c2) to head (e571361).
⚠️ Report is 3113 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #6610     +/-   ##
=========================================
- Coverage    82.8%    82.8%   -0.1%     
=========================================
  Files         849      849             
  Lines      379159   379165      +6     
=========================================
- Hits       314069   314024     -45     
- Misses      65090    65141     +51     

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[steveluscher]