apollo:client download-schema Add a flag to skip __ validation to be backward compatible with apollo codegen

We are in the process of moving our type generation process from codegen to cli. Our existing schema has __ in our fields. We worked around this by running introspect-schema using codegen replacing __ and generate the types from the schema.json without the __. The codegen introspect-schema used to work even if the schema had __. Trying the same thing with cli throws an error "Name "__metadata" must not begin with "__", which is reserved by GraphQL introspection" when running the download-schema command. I couldn't find a way to skip this validation. Can we add a flag to skip this validation to be compatible with the codegen behaviour?

Start at the download-schema command and trace where the "Name must not begin with __" validation error is raised. Check existing command options and validation tests, if present; done means an explicit flag skips this validation for legacy schemas while the default behavior remains unchanged.

Written by the indexing model from the issue text.