ci: add Reviewpad action

[marcelosousa]

Description

This pull request adds the Reviewpad action to the repository to start automating workflows around pull requests without adding a new GitHub action to each automation.

The automation added are specified in the .github/reviewpad.yml file:

• Add a label depending on the size of the pull request;
• Add a label depending on the modifications (replaces the existing labeler action - FYI: that action is only active in pull requests which means that it wouldn't run on contributions from forks and the extra permissions are not necessary);
• Automatically writes a welcome message to new contributors to the project
• Checks if there the history of a pull request is linear; if it is not, then adds a warning in the pull request as a comment.

Would be great to get some feedback on this!