add ability to provide custom Axios instance

[opyate]

What problem does the pull request solve?

Adds the ability to specify your own Axios client.

This is useful, because you might have a client with custom interceptors.

Checklist

• [x] I’ve used the pull request template
• [x] I’ve written unit tests for these changes
• [x] I’ve updated the documentation in
  • notifications-tech-docs repository
  • CHANGELOG.md
• [x] I’ve bumped the version number in
  • package.json
• [x] I've added new environment variables in
  • CONTRIBUTING.md
  • notifications-node-client/scripts/generate_docker_env.sh