[Go SDK]: Enable fileio.MatchContinuously to emit duplicate file if modified

[johannaojeling]

Updates the fileio.MatchContinuously transform to support emission of duplicate matches if a file has been modified since it was last observed. The behavior is enabled by passing the fileio.MatchDuplicateAllowIfModified() option. Fixes #26523.

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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]

Thanks for your patience. I was on vacation towards the end of last week, and just caught up on things. Looking at this now.

[lostluck]

LGTM! Thank you.

Sorry for the delay. Some critical distractions occurred after my initial look.

[lostluck]

Not in this PR, but we could consider putting this as a helper function in the filesystem package instead, to allow more reasonable re-use and consistency in the implementation:

Eg. We implement copy, but if there's a Copier implementation, it can be used instead.
https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/io/filesystem/util.go#L55

We'd probably want a "creation time" too though when we do that, so the default isn't the zero time. Not critical at this stage though.

[lostluck]

Oh dang, I will send a quick fix for this, but it looks like the time range might be incorrectly set for the local unit test:

Caught in another CL.

-- FAIL: TestLocal_lastModified (0.00s)
    local_test.go:177: LastModified("/tmp/TestLocal_lastModified2985575980/001/file.txt") = 2023-05-15 21:32:42.998867243 +0000 UTC, want in range [2023-05-15 21:32:43 +0000 UTC, 2023-05-15 21:32:43.004302634 +0000 UTC m=+0.001527498]
FAIL
	github.com/apache/beam/sdks/v2/go/pkg/beam/io/filesystem/local	coverage: 77.4% of statements
FAIL	github.com/apache/beam/sdks/v2/go/pkg/beam/io/filesystem/local	0.036s