refactor: extract flattenServiceAccountKey from Read function

[vr-ibm]

Extracts the field-setting logic from resourceGoogleServiceAccountKeyRead into a shared flattenServiceAccountKey function. This enables reuse by the upcoming list resource (#18242) without duplicating code.
No behavior change, pure refactor.

Details

• Extracts flattenServiceAccountKey from the Read function
• Read function now calls the shared flattener
• List resource PR (Add google_service_account_key list resource #18242) will call this same function, eliminating duplicate flatten logic

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 044e576:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	1 file changed, 5 insertions(+), 1 deletion(-)
google-beta provider	View Diff	1 file changed, 5 insertions(+), 1 deletion(-)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
183	152	31	0

Affected Service Packages

• resourcemanager

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