Add fs_preserveFieldCasing attribute

[joncham]

This enables overriding the behavior of --normalize-field-names at the field, struct, or table level.

This PR attempts to implements a solution for issue #305 . I did make the attribute a bit more verbose as fs_preserveFieldCasing but am happy to use the original suggestion of fs_preserveCase as well.

[joncham]

@jamescourtney I don't seem to have permission to add reviewer but FYI. cc @vvuk

[jamescourtney]

Thanks for this! I always appreciate contributions. I'll try to figure out how to get @vvuk added as a contributor. In the meantime, I will take a look at this, but I'll need you to be a little patient as real-life obligations prevent me from spending too much time on FlatSharp these days. Thanks again for the contribution -- I'll get back to you with my feedback in a few days.

[codecov[bot]]

Codecov Report

Merging #306 (7b85fb9) into main (f37672c) will decrease coverage by 0.02%. The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main     #306      +/-   ##
==========================================
- Coverage   95.75%   95.73%   -0.03%     
==========================================
  Files         114      114              
  Lines        7806     7816      +10     
  Branches      740      746       +6     
==========================================
+ Hits         7475     7483       +8     
  Misses        229      229              
- Partials      102      104       +2     

Impacted Files	Coverage Δ
src/FlatSharp.Compiler/MetadataKeys.cs	100.00% <ø> (ø)
src/FlatSharp.Compiler/FlatSharpCompiler.cs	90.80% <77.77%> (-0.53%)	:arrow_down:
...tSharp.Compiler/SchemaModel/FlatSharpAttributes.cs	97.50% <100.00%> (+0.03%)	:arrow_up:
...Compiler/SchemaModel/MutableFlatSharpAttributes.cs	83.33% <100.00%> (+0.72%)	:arrow_up:

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[vvuk]

@jamescourtney thanks -- @joncham and I work together, so you may see PRs for Unity integration coming from both of us over the next little while.

[joncham]

@jamescourtney do you want a followup PR with different naming?

[jamescourtney]

@joncham -- I think it's fine. I may change it to fs_literalName, but you seem to have bigger fish to fry :)