Fix errors in project retries, add billinginfo retries

[modular-magician]

Upstreams #5578

This change affects two areas:

1. When the result of a Projects.Update(...) request is an error, the returned
   'project' value is 'nil'. Previously, this value was stored in 'p' which
   was then dereferenced when retrieving 'p.ProjectId' on the subsequent retry.
   This fix only allows 'p' to be overwritten if the Projects.Update(...)
   request is successful.
2. Requests to the BillingInfo service could fail due to transient HTTP timeouts
   (usually TLS handshake negotiation timeout) and for throttling (HTTP
   429). Adding retries addresses this.

Release Note Template for Downstream PRs (will be copied)

resourcemanager: fixed retry behavior for updates in `google_project`, added retries for billing metadata requests

Derived from GoogleCloudPlatform/magic-modules#3091

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