Add Intermediate-Context header support for Talaria message metadata

[ChaitanyaSingla]

This PR introduces the intermediate-context header as an optional field in the metadata preparation flow for both online and offline messages sent by Talaria.

Key Changes:

1. Added support for intermediate-context header in message metadata construction.
2. Updated relevant structs and interfaces to include the new header.

Purpose:
The intermediate-context header allows for carrying any extra information that may be required by the receiving systems.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.69%. Comparing base (21f6a98) to head (9b82d10).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
device/mocks.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1160      +/-   ##
==========================================
+ Coverage   81.16%   81.69%   +0.53%     
==========================================
  Files         150      150              
  Lines        8791     7216    -1575     
==========================================
- Hits         7135     5895    -1240     
+ Misses       1466     1133     -333     
+ Partials      190      188       -2     

Flag	Coverage Δ
unittests	81.69% <60.00%> (+0.53%)	⬆️

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