[SPARK-8740] [PROJECT INFRA] Support GitHub OAuth tokens in dev/merge_spark_pr.py

This commit allows `dev/merge_spark_pr.py` to use personal GitHub OAuth tokens in order to make authenticated requests. This is necessary to work around per-IP rate limiting issues.

To use a token, just set the `GITHUB_OAUTH_KEY` environment variable.  You can create a personal token at https://github.com/settings/tokens; we only require `public_repo` scope.

If the script fails due to a rate-limit issue, it now logs a useful message directing the user to the OAuth token instructions.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7136 from JoshRosen/pr-merge-script-oauth-authentication and squashes the following commits:

4d011bd [Josh Rosen] Fix error message
23d92ff [Josh Rosen] Support GitHub OAuth tokens in dev/merge_spark_pr.py