add Targets factories to ease {maldipickr} workflow

[cpauvert]

This PR is to draft functions to facilitate reproducible and trustworthy workflow development {maldipickr} (using {targets}).

We invite readers unfamiliar with {targets} to read the short and well-written walk-through "How to use {targets}" to understand how {targets} help analysts "coordinate the pieces of computationally demanding analysis projects" and "skips costly runtime for tasks that are already up to date" (source: https://cran.r-project.org/package=targets).

These helpers in {maldipickr} are called {targets} factories, which means they return target objects or lists of target objects.

[codecov]

Codecov Report

Attention: Patch coverage is 56.66667% with 52 lines in your changes are missing coverage. Please review.

Project coverage is 89.40%. Comparing base (62ecd77) to head (875e2a2).

Files	Patch %	Lines
R/tar_pick_with_similarity.R	0.00%	52 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   96.67%   89.40%   -7.27%     
==========================================
  Files          17       20       +3     
  Lines         541      661     +120     
==========================================
+ Hits          523      591      +68     
- Misses         18       70      +52     

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[cpauvert]

This PR includes commits from what should have been a feature branch that I'm struggling to integrate properly. Even if I tried to find non-cherry-pick solutions with merge, I could not get it right: https://devblogs.microsoft.com/oldnewthing/20180323-01/?p=98325 So I'll close, rename the branch and create two proper feature branches: for gather_spectra_stats and targets-factories