Make map/broadcast a bit more customizable

[mtfishman]

Followup to #150. Make blockwise map/broadcast a bit more customizable by allowing customizing how unstored blocks get constructed. This is helpful for use cases like having blocks that are Kronecker arrays, since it is helpful to construct unstored blocks with Kronecker structures, which this PR enables by overloading FillArrays.fillsimilar.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.97%. Comparing base (b83242d) to head (0e9a478).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/blocksparsearrayinterface/map.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   73.97%   73.97%           
=======================================
  Files          36       36           
  Lines        1875     1875           
=======================================
  Hits         1387     1387           
  Misses        488      488           

Flag	Coverage Δ
docs	7.80% <0.00%> (ø)

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