Allow built in scalar overrides

[obmarg]

#711 highlighted that some GraphQL servers don't stick precisely to the spec, particularly around integers.

It's currently possible to work around this with newtypes, but I would like users to be able to just declare that i64 is acceptable in Int position. Similarly, if someone wants to use an alternative type in place of String (SmolStr for example) that should be possible.

The main obstacle to this is that we use built in types as type markers for these, so the orphan rule prevents you from doing impl IsScalar<i32> for any types that you don't own. If we updated the schema to output dedicated type markers for all the built in scalars that should make this possible.

Technically a breaking change, but probably not one that would affect most users.

Also want to make sure to document this when I get to it.