remove Debug implementation for secret values

[tomleavy]

Description of changes:

The pretty_bytes function allows for a user to specify raw byte output, which a user can accidentally use to log secret values. This PR removes the pretty_bytes function from structs or fields that contain secrets

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.68%. Comparing base (0e602b4) to head (65aedea).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
mls-rs-crypto-traits/src/kem.rs	0.00%	4 Missing ⚠️
mls-rs-core/src/crypto.rs	0.00%	3 Missing ⚠️
mls-rs/src/client_builder.rs	0.00%	3 Missing ⚠️
mls-rs/src/group/key_schedule.rs	0.00%	3 Missing ⚠️
mls-rs-core/src/group/group_state.rs	0.00%	1 Missing ⚠️
mls-rs-core/src/psk.rs	0.00%	1 Missing ⚠️
mls-rs-core/src/secret.rs	0.00%	1 Missing ⚠️
.../src/group/ciphertext_processor/sender_data_key.rs	0.00%	1 Missing ⚠️
mls-rs/src/group/epoch.rs	0.00%	1 Missing ⚠️
mls-rs/src/group/proposal.rs	0.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #318      +/-   ##
==========================================
+ Coverage   88.50%   88.68%   +0.17%     
==========================================
  Files         182      182              
  Lines       29002    28944      -58     
==========================================
  Hits        25668    25668              
+ Misses       3334     3276      -58     

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