Add extras to Layer's metadata

[andy-sweet]

This adds a key to the top-level of the Layer.metadata dictionary, which is used to store a custom dataclass that stores the extra metadata this plugin cares about that napari can't currently handle.

The reason for adding this is to be able to write reader and writer plugin contributions, rather than rely on opening file dialogs from the widget.

Changes to widgets cause these extra metadata values to be updated. However, changes in the metadata do not currently cause widget values to be updated - that will be handled as part of writing a plugin reader.

Closes #14

[codecov-commenter]

Codecov Report

Merging #17 (4e4e844) into main (4a14c16) will decrease coverage by 0.07%. The diff coverage is 89.24%.

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@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   89.41%   89.35%   -0.07%     
==========================================
  Files          10       11       +1     
  Lines         586      742     +156     
==========================================
+ Hits          524      663     +139     
- Misses         62       79      +17     

Impacted Files	Coverage Δ
src/napari_metadata/_model.py	84.26% <84.26%> (ø)
src/napari_metadata/_space_units.py	95.00% <85.71%> (-5.00%)	:arrow_down:
src/napari_metadata/_time_units.py	94.73% <85.71%> (-5.27%)	:arrow_down:
src/napari_metadata/_axes_name_type_widget.py	96.20% <94.73%> (-0.47%)	:arrow_down:
src/napari_metadata/_tests/test_widget.py	99.50% <100.00%> (+0.05%)	:arrow_up:
src/napari_metadata/_widget.py	79.88% <100.00%> (+1.89%)	:arrow_up:

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