Add support for Beam schemas

[kellen]

https://beam.apache.org/documentation/programming-guide/#schema-definition

[codecov]

Codecov Report

Attention: Patch coverage is 98.22222% with 4 lines in your changes missing coverage. Please review.

Project coverage is 95.50%. Comparing base (c373085) to head (50f67ec).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
shared/src/main/scala/magnolify/shared/Time.scala	95.31%	3 Missing ⚠️
beam/src/main/scala/magnolify/beam/RowType.scala	98.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
+ Coverage   95.23%   95.50%   +0.27%     
==========================================
  Files          51       56       +5     
  Lines        1845     1980     +135     
  Branches      160      186      +26     
==========================================
+ Hits         1757     1891     +134     
- Misses         88       89       +1     

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[clairemcginty]

looks good! only a few minor comments

[RustedBones]

we've used TableRow type for BQ, we should use RowType here