Fix all tour examples from part 1

[Dal-Papa]

Fix all the Go examples for the Tour of Beam all the way until "Core Transformations". A subsequent PR will fix the rest of the examples.

beam.Init was missing on all the files making it impossible to run in the Playground.
Also, go fmt format wasn't respected on a lot of the files.

This fixes #30477 (partly)

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[Dal-Papa]

R: @lostluck

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

[lostluck]

Only just saw this today. The original ping happened while I was on a boat. Thank you for your patience. Taking a look now.

[lostluck]

Well that was straightforward. Thank you very much for going ahead and fixing these!