[Go SDK]: Infer field names from struct tags

[johannaojeling]

Implements a function structx.InferFieldNames which takes a struct type and a tag key, and returns a slice of field names based on this. Fixes #24465.

Updates bigqueryio.Read and spannerio.Read to make use of structx.InferFieldNames when constructing select statements.

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[codecov]

Codecov Report

Merging #24473 (7d3d0a9) into master (ec2a07b) will increase coverage by 0.00%.
The diff coverage is 88.23%.

@@           Coverage Diff           @@
##           master   #24473   +/-   ##
=======================================
  Coverage   73.36%   73.37%           
=======================================
  Files         718      719    +1     
  Lines       97165    97185   +20     
=======================================
+ Hits        71287    71310   +23     
+ Misses      24531    24528    -3     
  Partials     1347     1347           

Flag	Coverage Δ
go	51.64% <88.23%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
sdks/go/pkg/beam/io/spannerio/spanner.go	50.94% <0.00%> (-1.65%)	⬇️
sdks/go/pkg/beam/io/bigqueryio/bigquery.go	9.89% <72.72%> (+4.14%)	⬆️
sdks/go/pkg/beam/util/structx/struct.go	100.00% <100.00%> (ø)

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[johannaojeling]

R: @lostluck

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[lostluck]

LGTM. Thank you for clearing out that TODO!

Please note that neither of those two IOs are currently incredibly performant. They work, but they are unable to scale to multiple machines presently. I go into why a bit in a recent reply: #24364 (comment)

[lostluck]

Run Go PostCommit

[johannaojeling]

LGTM. Thank you for clearing out that TODO!

Please note that neither of those two IOs are currently incredibly performant. They work, but they are unable to scale to multiple machines presently. I go into why a bit in a recent reply: #24364 (comment)

Thanks for the reference! I was thinking of developing a new IO connector and will bear it in mind