Add/expose all supported props to BootstrapVue wrapper components

many of our wrapper components auto-pass props to the underlying BV components. There are some downsides to this:

1. in vue test, we have to use vm.attributes('variant') rather than vm.props('variant')
2. Reading gitlab-ui code doesn't tell you what props we support
3. We kinda implicitly support everything that the underlying library does, so if there's an upstream breaking change we won't know until it breaks in main gitlab repo