Add dependabot.yml to automate package version upgrades

[jenny-s51]

Closes #818

[google-oss-prow]

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[andyatmiami]

/hold

to proceed with this PR - we want the following requirements satisfied:

• PoC / functional example of how we can configure dependabot to work across the multiple non-default branches we have in kubeflow/notebooks while also reasonably aggregating related dependencies
• e2e tests in place that cover common usage

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