Add Ability For Users to Self-Assign Issues

It would be nice if a user could self assign issues to themselves and have parity with other dapr repos where they may self assign issues.

Steps
That entails having a dapr_bot.js file, [like this one from dapr/dapr](https://github.com/dapr/dapr/blob/master/.github/scripts/dapr_bot.js) but simplified with only the ability to run /assign. [See here for example on how to have the /assign command](https://github.com/dapr/dapr/blob/master/.github/scripts/dapr_bot.js#L85-L89). This would entail needing the appropriate [dapr-bot.yml file](https://github.com/dapr/dapr/blob/master/.github/workflows/dapr-bot.yml), but with the appropriate changes based on it being a different repository with different maintainers. Please [ask in discord](https://discord.com/invite/ptHhX6jc34) or on the PR to find out who the appropriate maintainers are.

Also, please [update the the daprbot documentation](https://docs.dapr.io/contributing/daprbot/) in [dapr/docs](https://github.com/dapr/docs) noting that this repo now has the /assign command available.

As a final step, please link to the [daprbot documentation](https://docs.dapr.io/contributing/daprbot/) inside the contributing.md file so that users know what commands they can run in this repo.

[inspiration for simple implementation can be found here](https://github.com/dapr/java-sdk/blob/master/.github/workflows/dapr_bot.yml#L24)