Add optional workspace for allocation-free CHOLMOD solves

[lorenzogambichler]

Addresses #726

Adds optional preallocated workspace, so repeated solves against fixed sparse Cholesky factorization can reuse internal CHOLMOD cholmod_solve2 Y and E buffers. This eliminates any julia-sided allocations per solve (should reduce to 16 bytes total, as tested). The default solve path remains unchanged, since the workspace is fully optional. I am open to any alternative approaches or suggestions.

API:

F = cholesky(A)
ws = CholmodWorkspace()
for b in rhs
    ldiv!(x, F, b, ws)
end

Also added a testset similar to that of standard ldiv! above it with appropriate changes.

[codecov]

Codecov Report

❌ Patch coverage is 88.09524% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.34%. Comparing base (3f9b6fb) to head (5923f2f).

Files with missing lines	Patch %	Lines
src/solvers/cholmod.jl	88.09%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #727      +/-   ##
==========================================
- Coverage   84.39%   84.34%   -0.06%     
==========================================
  Files          13       13              
  Lines        9371     9413      +42     
==========================================
+ Hits         7909     7939      +30     
- Misses       1462     1474      +12     

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[lorenzogambichler]

The failed win x86/x64 nightly checks seem to be a dependency resolution fail, so unrelated to this PR from what I can tell.