[Go SDK] Stream decode values in single iterations

[lostluck]

Fixes #22900

This adds one time use stream decoders for use whenever we know reiterations won't occur (see issue).

Adds tests, and improves test coverage with a datasource tests for the various iterator handlers (on demand and fixed), unit tests for the on demand decode streams, a new test for pardo short reads, and a primitives test for GBK short reads.

Fixes previous benign issue where value iteration streams were not being closed after the processing DoFn finished, and test for same. Since we always fully read the byte stream for a value, we never ran into a short read problem before. Streaming decode short reads on the data side would cause problems that don't occur with short reads for Side Inputs.

There's a followup task for optimizing short reads when the coder being used is length prefixed, filed in #22901.

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[codecov]

Codecov Report

Merging #22904 (4b6ace8) into master (c866771) will increase coverage by 0.01%.
The diff coverage is 80.68%.

@@            Coverage Diff             @@
##           master   #22904      +/-   ##
==========================================
+ Coverage   73.88%   73.90%   +0.01%     
==========================================
  Files         713      713              
  Lines       94321    94464     +143     
==========================================
+ Hits        69691    69813     +122     
- Misses      23342    23354      +12     
- Partials     1288     1297       +9     

Flag	Coverage Δ
go	51.36% <80.68%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/runtime/exec/datasource.go	66.30% <76.31%> (+0.74%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/fullvalue.go	83.59% <81.73%> (-2.29%)	⬇️
sdks/go/pkg/beam/core/runtime/exec/fn.go	70.76% <100.00%> (+2.44%)	⬆️

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[damccorm]

Left a couple nits, overall LGTM - this is a good approach

[lostluck]

Thanks for the review!