Serialize settings for size attributes

[Carifio24]

Serialization of size attributes currently doesn't work. We have a special column name (SIZE_COLUMN_NAME) designated for storing sizes in the serialization, but that column name is never actually used in the layer's data table. This PR updates the serialized state to match the settings messages that are dispatched here.

I'm not sure how often serialization is used, but one intended use case would be glue-viz/glue-wwt#62, as using size attributes is a common operation in glue.

[pkgw]

Hmm. It's been a little while since I've thought about this aspect of things, so I want to be careful, but I think that this is correct. The key is that pywwt used to compute the sizes for WWT, and used the magic column name for those computed sizes. But in 9c358cf we started relying on built-in support for the engine and I think that we stopped needing to use the special column. Does that jibe with your understanding?

[Carifio24]

Yep! That's how I had understood things as well.

[Carifio24]

The CI failure looks like a transient timeout thing? But I suspect it would've needed another run anyways to pick up the new CI updates

[pkgw]

I think you'll need to rebase this onto the latest master (or merge it in to your branch) for it to pick up the CI fixes.

[Carifio24]

Just rebased

[codecov]

Codecov Report

Merging #345 (6167f62) into master (d76265b) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 6167f62 differs from pull request most recent head 7efa3eb. Consider uploading reports for the commit 7efa3eb to get more accurate results

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
+ Coverage   56.12%   56.17%   +0.04%     
==========================================
  Files          25       25              
  Lines        2833     2836       +3     
==========================================
+ Hits         1590     1593       +3     
  Misses       1243     1243              

Impacted Files	Coverage Δ
pywwt/layers.py	75.69% <100.00%> (+0.10%)	⬆️

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