Parametrize VType in KrylovSubspace constructor

[mcarmesin]

Checklist

• [ ] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

Allow for overriding the type of the basis vector matrix V used for the KrylovSubspace. V has to be a GPUArray if arnoldi!(Ks,A,b) is executed for b being GPUArray.