Add usage_period fields to billing_budget resource

[modular-magician]

Adds in the usage_period fields calendar_period and custom_period to the google_billing_budget resource.

Fixes hashicorp/terraform-provider-google#10753
Fixes hashicorp/terraform-provider-google#9196

Also corrected some documentation errors with correct wording from public docs for creditTypes description. There are some additional documentation fixes needed, such as hashicorp/terraform-provider-google#9458

This PR is in draft as there are some outstanding issues I need assistance with:

• Removing the budget_filter block will not reset filters to default values. Conversely, if default values are set or computed for calendar_period, errors can occur when trying to change from calendar_period to custom_period, as terraform won't unset the calendar_period property. Perhaps a custom_flatten or custom_expand is needed but nothing I have tried has worked thus far.
• Validation for integer values in custom_period

Marking this PR as draft to hopefully get some feedback on the above items.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

billing: Added `calendar_period` and `custom_period` fields to `google_billing_budget`

Derived from GoogleCloudPlatform/magic-modules#6099