Add Content-Type header to Agent responses

[simonmarty]

• Provide a prefer-post-quantum crate feature
• Add Content-Type header

Issue #, if available:

Merge #78 first

Description of changes:
Useful for pretty-printing agent responses in browsers and API test tooling.

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

Attention: Patch coverage is 75.00000% with 13 lines in your changes missing coverage. Please review.

Project coverage is 41.87%. Comparing base (8f399b0) to head (8986fbf).

Files with missing lines	Patch %	Lines
aws_secretsmanager_agent/src/server.rs	71.73%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   42.06%   41.87%   -0.20%     
==========================================
  Files          14       14              
  Lines        8969     9039      +70     
==========================================
+ Hits         3773     3785      +12     
- Misses       5196     5254      +58     

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