Add metadata key to ImageInterface

[h-mayorquin]

A movement in the direction of #269 that will allow adding more than one container.

[pauladkisson]

I left a very small note about future warning vs deprecation warning. Otherwise, looks great.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.14%. Comparing base (b8afb7a) to head (91ee665).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/neuroconv/datainterfaces/image/imageinterface.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1439      +/-   ##
==========================================
- Coverage   87.14%   87.14%   -0.01%     
==========================================
  Files         145      145              
  Lines        9623     9635      +12     
==========================================
+ Hits         8386     8396      +10     
- Misses       1237     1239       +2     

Flag	Coverage Δ
unittests	87.14% <84.61%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
...c/neuroconv/datainterfaces/image/imageinterface.py	89.60% <84.61%> (-0.67%)	⬇️

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