feat: Validate workspace specification before accessing parts of it

[alexander-held]

Description

The validation of a workspace specification against the schema currently takes place after some aspects of the workspace are already accessed in the constructor of _ChannelSummaryMixin. This can lead to unintuitive error messages, which should instead be caught by the schema validation. This PR re-orders the logic such that validation takes place first, ensuring that the relevant information is available for subsequent steps (assuming validation succeeds).

resolves #1952

Checklist Before Requesting Reviewer

• 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

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Validate the workspace specification in Model construction before any spec information
  can be accessed in the constructor of _ChannelSummaryMixin.
* Add test that checks for an pyhf.exceptions.InvalidSpecification being raised, indicating
  that the invalid model spec has been caught before a KeyError would be raised later.

[codecov]

Codecov Report

Merging #1953 (90e82a2) into master (fd589ef) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1953   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          69       69           
  Lines        4439     4439           
  Branches      748      748           
=======================================
  Hits         4362     4362           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	25.81% <0.00%> (ø)
doctest	61.02% <100.00%> (ø)
unittests-3.10	96.14% <100.00%> (ø)
unittests-3.7	96.13% <100.00%> (ø)
unittests-3.8	96.17% <100.00%> (ø)
unittests-3.9	96.19% <100.00%> (ø)

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

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

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[matthewfeickert]

Thanks @alexander-held! We very much appreciate your contributions. :)