Websocket: Validate UTF-8 in text payloads

[graebm]

Issue:
Websocket was not validating text payloads, as required by RFC-6455 Section 5.6

Description of changes:
Now it is!

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Base: 79.50% // Head: 79.44% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (89715c2) compared to base (9bd58ca).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   79.50%   79.44%   -0.07%     
==========================================
  Files          27       27              
  Lines       11912    11934      +22     
==========================================
+ Hits         9471     9481      +10     
- Misses       2441     2453      +12     

Impacted Files	Coverage Δ
source/websocket.c	82.66% <100.00%> (+0.02%)	⬆️
source/websocket_decoder.c	100.00% <100.00%> (ø)
source/h2_connection.c	82.83% <0.00%> (-1.16%)	⬇️
source/connection_manager.c	88.21% <0.00%> (+0.57%)	⬆️

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