Add an `otelcol.receiver.datadog` component

[ptodev]

PR Description

Introducing a new otelcol component which receives Datadog metrics and traces.

This component is initially introduced at the experimental level.

Which issue(s) this PR fixes

Fixes #889

PR Checklist

• [x] CHANGELOG.md updated
• [x] Documentation added
• [x] Tests updated
• [x] Config converters updated