fix: didcomm message forwarding

[Patrik-Stas]

• Previous refactoring PR refactor: encryption envelope #1070 introduced an error such that to field in the forward message ended up string containing JSON array ("to": "[\"foobar\"]"), rather than correctly just the key ("to": "foobar")
• This PR fixes the issue
• The issue was detected as mediator tests started to fail
• This PR is to be followed by another PR restoring encryption envelope tests

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (5a42a2a) 0.05% compared to head (3a2da3d) 0.05%.

Files	Patch %	Lines
aries/aries_vcx/src/utils/encryption_envelope.rs	0.00%	13 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1081      +/-   ##
========================================
- Coverage   0.05%   0.05%   -0.01%     
========================================
  Files        471     471              
  Lines      24015   24019       +4     
  Branches    4292    4295       +3     
========================================
  Hits          13      13              
- Misses     24001   24005       +4     
  Partials       1       1              

Flag	Coverage Δ
unittests-aries-vcx	0.05% <0.00%> (-0.01%)	⬇️

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[xprazak2]

Thanks!