Allow other providers

[odama626]

apollo should be removed as a dependency of @flatbread/core and extracted into it's own package @flatbread/provider-apollo the flatbread package should be trimmed down to only contain the cli, and a new flag should be added to allow specifying which provider to use when running the flatbread command

options for the flatbread package -- should implement both of possible

• Preferably: add a provider field to the config that accepts a provider that the cli will use
• add a -p, --provider flag to the cli to specify which provider to use with the cli

[tonyketcham]

I agree it'd be best to have the provider declared in the config so that third-party providers can easily be registered.

We might be able to get the provider out of the config before passing it to generateSchema, and then feed the remaining config into generateSchema.