Move dqflags from romancal to roman_datamodels and make them enums.

[WilliamJamieson]

When working on jump detection for romancal I noticed that the dqflags were stored in romancal instead of roman_datamodels. The dqflags being directly part of the pipeline instead of the datamodels package proved to be an issue for JWST as some tools wished to not have a direct dependency on the JWST pipeline simply to open and interpret existing output files, see spacetelescope/stdatamodels#134.

This PR begins the process of moving the dqflags from romancal to roman_datamodels by "duplicating" them into roman_datamodels. Note that at this time, romancal will still be referencing its internally defined flags, an update to this effect can be made later to romancal which is seamless to the development process.

Additionally, this PR transitions the dqflags from using simple dictionaries to defining them using an Enum as I discussed briefly with @schlafly. This adds two useful things:

1. It protects the defined values of the dqflags from accidentally being changed at run time. The current use of a dict is susceptible to the possibility of a module importing the dqflags updating a flag's value, which would change that value globally.
2. Allows for . access to the flag values e.g. pixel.GOOD while still retaining the dictionary like access e.g. pixel["GOOD"]. The . access is nice because it enables static checkers to "flag" when something like a typo in a flag's name occurs and code completion engine's to suggest the list of possible flags.

Checklist

• Added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bd88351) 97.48% compared to head (74d8aa8) 97.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #293      +/-   ##
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+ Coverage   97.48%   97.56%   +0.07%     
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  Files          28       30       +2     
  Lines        2704     2788      +84     
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+ Hits         2636     2720      +84     
  Misses         68       68              

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[PaulHuwe]

LGTM. Be sure to get approval on the RCAL companion ticket before merging this.

[schlafly]

This looks good to me. It's good to have this in roman_datamodels rather than romancal so that people can use the flags without bringing in romancal as a dependency.