refactor automerge to support merge for protected branch

[pxLi]

Previous auto-merge workflow only works for auto-merge into an unprotected branch, as how cudf's automerge does.
But this is not reasonable when we switch branch-0.3 as default branch, so I added auto-approve for this automation workflow, and it will try approving auto-merged pr if possible.

demo PR: pxLi#153

NOTE: a new secret NVAUTO_TOKEN has to be added to repo since github requires a different account to approve the PR. nvauto is our automation account which has already been invited to spark-rapids repo.

[pxLi]

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[jlowe]

only works for auto-merge into an unprotected branch, as how cudf's automerge does

Is that really the case? I find it very surprising that cudf's repo would leave the new development branch unprotected. Also my personal experience with cudf during auto-merge periods is that the new branch still requires CI checks and approvals before developers can merge to the branch, yet GPUtester can still merge the PRs without approval.

[pxLi]

only works for auto-merge into an unprotected branch, as how cudf's automerge does

Is that really the case? I find it very surprising that cudf's repo would leave the new development branch unprotected. Also my personal experience with cudf during auto-merge periods is that the new branch still requires CI checks and approvals before developers can merge to the branch, yet GPUtester can still merge the PRs without approval.

yeah, they keep the CI check but added the [skip ci] thing for auto-merge scenario to pass the CI checks. I did the same thing for our new blossom-ci pipeline PR #642

[pxLi]

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