[Go SDK]: Implement fileio.MatchContinuously transform

[johannaojeling]

Fixes #26186 and adds a fileio.MatchContinuously transform to the Go SDK.

I have tested the transform manually on Dataflow and Flink, which seem to be the only runners that currently support all of watermark estimation, self-checkpointing and state provisioning, that this transform relies on. I have not added any unit tests for the same reason, and there were limitations with running integration tests with some of these features for Dataflow and Flink so I have skipped that as well.

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[johannaojeling]

I would like to also enable emitting a file only if it has been modified since the last observation, but it requires introduction of a new method to the filesystems for retrieving a file's last modified timestamp first (#26187) + attaching this info to the fileio.FileMetadata in fileio.matchFn, so I will do it at a later stage

[lostluck]

I'll review PR 26192 first then.

[codecov]

Codecov Report

Merging #26188 (81bd33f) into master (dc6616d) will decrease coverage by 9.93%.
The diff coverage is 8.33%.

❗ Current head 81bd33f differs from pull request most recent head 102ad04. Consider uploading reports for the commit 102ad04 to get more accurate results

@@            Coverage Diff             @@
##           master   #26188      +/-   ##
==========================================
- Coverage   81.06%   71.13%   -9.93%     
==========================================
  Files         469      787     +318     
  Lines       67187   103365   +36178     
==========================================
+ Hits        54463    73531   +19068     
- Misses      12724    28352   +15628     
- Partials        0     1482    +1482     

Flag	Coverage Δ
go	53.61% <8.33%> (?)

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Impacted Files	Coverage Δ
sdks/go/pkg/beam/io/fileio/read.go	79.31% <ø> (ø)
sdks/go/pkg/beam/io/fileio/match.go	48.10% <8.33%> (ø)

... and 330 files with indirect coverage changes

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[github-actions]

Reminder, please take a look at this pr: @riteshghorse @pabloem

[riteshghorse]

This is great, thanks for your contribution!

[riteshghorse]

R: @lostluck

[github-actions]

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[lostluck]

Ah foiled! We can fix the lack of tests a bit once state is added to Prism, since that's what's currently missing here.

[lostluck]

Happy to merge this in if there are no further question/disccusion (after one of us fixes the conflict in CHANGES.md of course!)

[lostluck]

LGTM. Happy to merge this in, unless you want to have the files modified bit come in first/same time. Let me know, and I'll merge as appropriate.

I don't know if anything from the Java/Python equivalents require timers though, which are almost ready to go in.

[johannaojeling]

Thank you both for reviewing!

The Java and Python equivalents don't use timers. Happy too add tests for this one later on when state is supported.

I can open a separate PR for the modified files part once #26192 has been merged so please go ahead and merge.

[lostluck]

SGTM. I'm probably over-indexing on the importance of garbage collecting state. It's unlikely to be a problem for any but the most egregious number of files.