Add save functions for user labware definitions and offsets

[btmorr]

overview

Adds save functions for user definitions and offset files in new labware definition format.

Fixes #796

changelog

• (feature) Add save functions for user definitions and offset files in new labware definition format

[codecov]

Codecov Report

Merging #799 into v3a will increase coverage by 0.04%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##              v3a     #799      +/-   ##
==========================================
+ Coverage   69.26%   69.31%   +0.04%     
==========================================
  Files         193      193              
  Lines        6231     6247      +16     
  Branches      191      191              
==========================================
+ Hits         4316     4330      +14     
- Misses       1852     1854       +2     
  Partials       63       63

Impacted Files	Coverage Δ
api/opentrons/data_storage/serializers.py	100% <100%> (ø)	⬆️
api/opentrons/data_storage/labware_definitions.py	90.9% <85.71%> (-1.4%)	⬇️

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[andySigler]

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