Send billing_project for every request when set

[modular-magician]

Upstream of #9636, making up for some churn in headers since then and fixing up some documentation that's become fairly stale

This means that more resources will send the project override header, requiring additional permissions on the quota project. I believe that's fine, and safe- the only way a user could run into issues is if they've enabled user_project_override, defined a billing_project, are using no resources that already supported a billing project (all MMv1 resources, 70%+ of total), and have not granted the appropriate permission to their credentials.

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)

provider: added support for `billing_project` across all resources. If `user_project_override` is set to `true` and a `billing_project` is set, the `X-Goog-User-Project` header will be sent for all resources.

Derived from GoogleCloudPlatform/magic-modules#5086

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