Support reviewers field in PullRequestInput for Gitea

[localleon]

I would like to add support for specifying reviewers in the Gitea driver. However, the go-scm library currently only allows passing the generic input *scm.PullRequestInput, which does not provide a way to specify reviewers (usernames or teams) for pull requests.

Feature request:

• Extend the scm.PullRequestInput struct (and any relevant API layers) to support a reviewers field.
• Ensure the reviewers field can be used for Gitea providers, following their respective API specifications.
• Update documentation and examples to show how to add reviewers when creating pull requests via go-scm.

This change will enable tools like updatecli to provide feature parity for reviewers when automating pull request creation for both GitHub and Gitea using this library!

Docs:

• https://gitea.com/api/swagger/#/repository/repoCreatePullRequest