fix serialization struggles in SMB transform API

[clairemcginty]

I fixed some of the serialization issues people have been seeing with the transform API when they try to extract the Read into a non lazy variable. I did this by eagerly converting the Read into a BucketedInput right away so we're not passing through non-serializable members of Read such as Schema/CodecFactory.

Tested it out in unit test, DirectRunner & DataflowRunner and the new API passes serialization each time 👍

I also updated the transform unit test to use the same assertions that the Sink does -- i.e. checking that the elements have all been correctly rehashed into the new bucketing scheme.

[codecov]

Codecov Report

Merging #3342 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3342   +/-   ##
=======================================
  Coverage   72.70%   72.70%           
=======================================
  Files         234      234           
  Lines        7708     7708           
  Branches      351      345    -6     
=======================================
  Hits         5604     5604           
  Misses       2104     2104           

Impacted Files	Coverage Δ
.../smb/syntax/SortMergeBucketScioContextSyntax.scala	31.52% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 552aa13...e652a8a. Read the comment docs.