Strongly typed cache and export options

[blampe]

Right now we only take raw strings. It would be preferable to take strong-typed input, with the option to provide raw strings if necessary.

Cache schema should follow https://docs.docker.com/build/cache/backends/

Exporters schema should follow https://docs.docker.com/build/exporters/

This is related to #969, since unknown values on any of these fields will also need to be handled.