compute: convert service_attachment_test.go file to .go.tmpl

[BBBmau]

This conversion is from #16493

the PR introduced a beta only field but the test file itself is a regular go file resulting in the nightly test running the field on both GA and beta instead of just beta.

Having it a .go.tmpl will allow us to prevent the test on the beta only field to be generated on google-beta

TeamCity Build which shows the error from using field in GA provider

Release Note Template for Downstream PRs (will be copied)

See Write release notes for guidance.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 1 insertion(+), 123 deletions(-))
google-beta provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[modular-magician]

Tests analytics

Total tests: 1367
Passed tests: 1277
Skipped tests: 90
Affected tests: 0

Click here to see the affected service packages

• compute

🟢 All tests passed!

View the build log

[aishwaryavora]

Thanks for the fix.

[aishwaryavora]

@BBBmau and @hao-nan-li should we merge this PR?