Feature Request: Add ability to validate types are correct

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

1. CI!
1. 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.

1. [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 \
-
```

Start with the `apollo client:codegen` CLI entry point and review how `--config`, `--outputFlat`, `--target`, and scalar options select generated files. Define the validation behavior around comparing generated output without writing it and returning a non-zero exit code when files differ. Confirm completion with a CI-style command and the existing codegen checks, if available.

Written by the indexing model from the issue text.