Add _upgrade and _add_gens for mpolys

[SoongNoonien]

This is a follow up of #2182 and it replaces #2192. As discussed in #2193 and #2192 respectively, I haven't exported the two new methods. For now they are very specialized just for the current usecase of the universal polynomial ring and what will be become the universal ring. In the future im_func from OSCAR could be moved to AbstractAlgebra to replace _upgrade. With im_func it would theoretically be possible to perform some computations in smaller rings. Currently, if we have the polynomials f=x and g=z in a universal polynomial ring, which was created with the only generator x and later extended by y and z, their sum would be computed by mapping f and g to a multivariate polynomial ring in x, y and z. But with im_func they could also be mapped to a multivariate polynomial ring in just x and z. With larger examples this could potentially decrease computation time.

The diff to #2182 shouldn't be breaking.

[codecov]

Codecov Report

❌ Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.95%. Comparing base (28070b1) to head (cbd60b4).

Files with missing lines	Patch %	Lines
src/generic/UnivPoly.jl	87.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2198      +/-   ##
==========================================
- Coverage   87.95%   87.95%   -0.01%     
==========================================
  Files         127      127              
  Lines       31787    31785       -2     
==========================================
- Hits        27957    27955       -2     
  Misses       3830     3830              

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