Add DCO requirements and updated code of conduct

[willtsai]

Issue reference: https://github.com/dapr/docs/issues/2039

Description

Add DCO requirements and updated code of conduct. Content inspired and borrowed from https://github.com/kedacore/keda/blob/main/CONTRIBUTING.md#developer-certificate-of-origin-signing-your-work

Issue reference

https://github.com/dapr/docs/issues/2039

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• [x] Code compiles correctly
• [x] Created/updated tests
• [x] Unit tests passing
• [x] End-to-end tests passing
• [x] Extended the documentation / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]
• [x] Specification has been updated / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]
• [x] Provided sample for the feature / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]