add botorch gp learners

[be-marc]

Thank you for contributing a learner to the mlr3 ecosystem.
Please make sure that:

• The added learner(s) are sufficiently tested
• All the CI tests are passing (including the CodeFactor)
• You ran devtools::document()
• You updated the NEWS.md field to include the addition of the learner
• You did not modify anything not related to the new learner
• You are listed as a contributor in the DESCRIPTION of the R package