[Transmissions] Add force and current interfaces

[jordan-palacios]

Follow up on #2310.

Same concept and approach, only this time we add force and current interfaces.

As before, adding these two new interfaces is backwards compatible. If force and current interfaces are not available then the contents from this PR do nothing.

[codecov]

Codecov Report

❌ Patch coverage is 85.36585% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.42%. Comparing base (bb87986) to head (d6a2549).

Files with missing lines	Patch %	Lines
...ansmission_interface/differential_transmission.hpp	68.42%	9 Missing and 3 partials ⚠️
...ude/transmission_interface/simple_transmission.hpp	62.50%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2588      +/-   ##
==========================================
- Coverage   89.45%   89.42%   -0.03%     
==========================================
  Files         147      147              
  Lines       16636    16752     +116     
  Branches     1400     1408       +8     
==========================================
+ Hits        14881    14980      +99     
- Misses       1218     1227       +9     
- Partials      537      545       +8     

Flag	Coverage Δ
unittests	89.42% <85.36%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
..._interface/test/differential_transmission_test.cpp	99.61% <100.00%> (+0.09%)	⬆️
...ission_interface/test/simple_transmission_test.cpp	100.00% <100.00%> (ø)
...ude/transmission_interface/simple_transmission.hpp	75.64% <62.50%> (-1.64%)	⬇️
...ansmission_interface/differential_transmission.hpp	77.65% <68.42%> (-2.91%)	⬇️

... and 1 file with indirect coverage changes

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[christophfroehlich]

is this needed?