feat: create shared reservation

[BigBlackWolf]

Description

Fixes #

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Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• compute/reservations/create_shared_reservation.go:17, tag compute_reservation_create_shared

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[telpirion]

question: why are we mocking the client?

[BigBlackWolf]

Hey! It was a request to do in this way, because without mocking it would be needed to set up one more project in CI
cc: @m-strzelczyk

[rsamborski]

q: Is this used only for mocking the client? Is there a way to make a sample fully standalone (it should initialise the client), but still mock the client when running tests?

See my example in Python (different sample): GoogleCloudPlatform/python-docs-samples#12598

[BigBlackWolf]

Yeah, it's done this way to allow for either real or mocked client. I tried to do it in a mocking way without modifying the sample, but in Go it couldn't be done as in Python or Java...
Go doesn't support object replacement in runtime, so the typical solution for that is to make an abstraction (interface) that matches the same signature. I tried to cover the gap in comments to explain how it can be done by the developer, who will run this sample, but agree that it doesn't seem ideal.
Perhaps I am missing something - I am open to any suggestions on how to improve it