Use explicit flag for diagnostic settings to make deployment of diagnostic settings deterministic

[celloza]

Description

Fixes #61

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✅ Bug fix (a change which fixes an issue)
🔲 Refactoring (code cleanup or optimisation)
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[johncollinson2001]

Looks good, can we add some tests to assert the validation rule?

Here's an example - setup a funky scenario, then add this variable to the expect_failures block:

az-backup/tests/integration-tests/backup_modules_blob_storage.tftest.hcl

Line 167 in 9f056f3

run "validate_retention_period" {

[johncollinson2001]

We'll need to update this e2e test to take account of the new config:

az-backup/tests/end-to-end-tests/diagnostic_settings_test.go

Line 80 in 9f056f3

"log_analytics_workspace_id": *externalResources.LogAnalyticsWorkspace.ID,

[johncollinson2001]

Also an update needed to this terraform test:

az-backup/tests/integration-tests/backup_vault.tftest.hcl

Line 81 in 9f056f3

run "configure_vault_diagnostics_when_enabled" {