feat: accept additional metadata in diff2term

[ven-k]

This can be useful in passing metadata defined upstream; for example units defined in MTK.

With this PR: k_t's unit will be 1.0 s⁻¹

using ModelingToolkit
using ModelingToolkit: t, D, get_unit, value, VariableUnit
@variables k(t)
get_unit(diff2term(value(D(k)), Dict(VariableUnit => get_unit(D(k)))))  # returns 1.0 s⁻¹

[ChrisRackauckas]

Needs tests

[ven-k]

Due to ParameterizedFunctions, MTK@8 is resolved in the downstream tests. Hence, the tests use Unitful and not the DynamicQuantities.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 76.10%. Comparing base (79c4e92) to head (d32e23d).
Report is 142 commits behind head on master.

Files	Patch %	Lines
src/utils.jl	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1104      +/-   ##
==========================================
- Coverage   77.07%   76.10%   -0.98%     
==========================================
  Files          32       35       +3     
  Lines        3533     3678     +145     
==========================================
+ Hits         2723     2799      +76     
- Misses        810      879      +69     

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