Feature Request: Add ability to validate types are correct

[justinanastos]

I'd like the ability to validate my codegen'ed types from the CLI for two reasons:

1. CI!

2. When I forget to update types on the fly, I'd like to be able to use something like git bisect to find exactly which commit I forgot to update the types in:

   git bisect run npx apollo client:codegen  \
       --config=[config-filename] \
       --passthroughCustomScalars  \
       --outputFlat  \
       --validate
   

I can think of two ways to handle this:

1. [Preferred] Add an option that generates types, compares to all files that will be written instead of actually writing, and then exit with a non-zero exit code if any files are incorrect.

2. [Stopgap?] Allow output to stdout instead of always writing to a file

   For example, allow a - param to signify that we want to write to stdout instead of a real file; possibly only allow such an option when --outputFlat is also specified.

   npx apollo client:codegen --config=[config-filename]  \
       --passthroughCustomScalars  \
       --outputFlat  \
       --target=typescript  \
       -