Implement buildASTSchema

The reference implementation has this function: https://github.com/graphql/graphql-js/blob/main/src/utilities/buildASTSchema.ts

This would allow for building a Schema from a string, which is asked for in https://github.com/graphql-go/graphql/issues/353

I already have a [WIP fork](https://github.com/graphql-go/graphql/compare/master...alex-lange:lange-add-build-schema?expand=1) that does this already, so I am adding this issue here mostly for reference :)