[Automation] Add github actions for releasing

[pierotibou]

Description

This PR can be reviewed more easily commit by commit. Most of the code comes from dd-trace-dotnet.
Changes (each line is a commit):

• Adds a version bump action that will update the version, changelog and open a PR for review
• Adds an action triggered when the version bump PR is merged. It pushes the ddtrace-x.y.z branch and generate the draft release
• Adds an action triggered on merged release to close the milestone

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.