PULL_REQUEST_TEMPLATE

Description

Fixes #

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

I have followed Sample Guidelines from AUTHORING_GUIDE.MD
README is updated to include all relevant information
Tests pass: nox -s py-3.9 (see Test Environment Setup)
Lint pass: nox -s lint (see Test Environment Setup)
These samples need a new API enabled in testing projects to pass (let us know which ones)
These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
Please merge this PR for me once it is approved