Add get_proposed_dt to custom integrators

[DanielDoehring]

This is required e.g. by the glm_callback when simulating 2D/3D MHD.

[github-actions]

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[ranocha]

Thanks

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (318e145) 96.24% compared to head (6c32269) 96.23%.

Files	Patch %	Lines
src/time_integration/methods_2N.jl	0.00%	2 Missing ⚠️
src/time_integration/methods_3Sstar.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1776      +/-   ##
==========================================
- Coverage   96.24%   96.23%   -0.01%     
==========================================
  Files         436      436              
  Lines       35297    35301       +4     
==========================================
  Hits        33969    33969              
- Misses       1328     1332       +4     

Flag	Coverage Δ
unittests	96.23% <0.00%> (-0.01%)	⬇️

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