Update google_billing_budget resource to add last_period_amount

[modular-magician]

Fixes hashicorp/terraform-provider-google#5245

This uses overrides for a boolean to transform it into an empty block, as the LastPeriodAmount field in the billingAccounts.budgets API doesn't currently have any configureable properties. See details in API documentation.

This workaround was mentioned by @danawillow in the above issue and is being used in BigTable AppProfile. I can also see it's now being used in the dns and secret manager resources.

In the Terraform example / test I've added somewhat of a disclaimer. I don't think this is typical, but felt this was needed to call out it's not a "good" example of how actually to set the threshold_percent value, and it could be confusing if not mentioned (people may assume 10.0 = 10% when it's actually 1000%).
My concern is if this is set to a more normal value (e.g. 1.0) it could trigger actual email alerts to Billing Admins when running tests.

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Release Note Template for Downstream PRs (will be copied)

billing_budget: added `last_period_amount` field to `google_billing_budget` to allow setting budget amount automatically to the last billing period's spend.

Derived from GoogleCloudPlatform/magic-modules#3860