reservation_affinity controls how instances or instances created from
templates consume Compute Engine resource reservations.

Fixes hashicorp/terraform-provider-google#4593

This hoists my PR from hashicorp/terraform-provider-google#7669 up to magic-modules so this effort is not on someone else to do.

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. – I ran the subset of tests relating to added functionality
• 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). - tests added
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know). - ran the specific subset of tests added for this functionality
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: added `reservation_affinity` to `google_compute_instance` and `google_compute_instance_template`

Derived from GoogleCloudPlatform/magic-modules#4335