Close hostfxr handle, skip creating unnecessary parameters

[elinor-fung]

• Call hostfxr_close in AppLoader instead of trying to free the handle
• Don't create/pass optional hostfxr_initialize_parameters when initializing
• Use StringMarshalling property on LibraryImport to set marshalling for all strings in the function

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[elinor-fung]

@kshyju this is some of the suggestions we sent over e-mail for the native host.

cc @vitek-karas

[fabiocav]

Thank you, @elinor-fung !