feat: Promote validate kwarg to top-level functions in pyhf.simplemodels

[phinate]

When doing experiments for differentiable models (#882), it's sometimes been easier to just skip validation with simplemodels, which I've been writing out specs for. This just reduces a bit of code overhead for me when I do that ;)

Made sure to highlight in the docstring that this should probably never be touched if one doesn't know why.

This is also partially because #1665 isn't fully done yet, which would probably make this redundant in some ways.

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• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

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For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add `validate` kwarg to pyhf.simplemodels.uncorrelated_background and
pyhf.simplemodels.correlated_background API. This allows expert users to avoid
validating their models in specific circumstances.
* Add Nathan Simpson to contributors list.

[kratsg]

Since master allows for custom specs - why not use a custom "catch-all" spec like type: object and no other details?

[codecov]

Codecov Report

Base: 98.24% // Head: 98.24% // No change to project coverage 👍

Coverage data is based on head (98d788a) compared to base (c68e43b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1858   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files          68       68           
  Lines        4378     4378           
  Branches      726      726           
=======================================
  Hits         4301     4301           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.58% <50.00%> (ø)
doctest	60.57% <100.00%> (ø)
unittests-3.10	96.13% <100.00%> (ø)
unittests-3.7	96.12% <100.00%> (ø)
unittests-3.8	96.16% <100.00%> (ø)
unittests-3.9	96.18% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/simplemodels.py	92.30% <100.00%> (ø)

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[matthewfeickert]

LGTM. 👍 Thanks for the PR and for your contributions @phinate (also thanks for your patience as this one got put off for some time)!