fix(federation): output key directive to entity sdl

[meskill]

Summary:
Briefly describe the changes made in this PR.

Issue Reference(s):
Fixes #... (Replace "..." with the issue number)

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.55%. Comparing base (459ef4e) to head (5ba920a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/config/transformer/subgraph.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2973      +/-   ##
==========================================
- Coverage   87.55%   87.55%   -0.01%     
==========================================
  Files         275      275              
  Lines       27256    27256              
==========================================
- Hits        23864    23863       -1     
- Misses       3392     3393       +1     

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