chore: refactor rover-client in pursuit of structured output

[EverlastingBugstopper]

This PR refactors rover-client into a more principled library that could be consumed by external applications. The main takeaway here is: no more codegen types in rover-client's api! everything has its own type now, there is an input and an output struct with stronger types, shared types are defined in rover-client and most have FromStr implementations that are now used directly in structopt, and we do not re-export any auto-generated types which means we will be able to swap out our graphql clients at will without worrying about breaking changes.

This lays the ground work for structured output, I will have a follow-up PR to add a --json flag that should be able to utilize all of the work that's gone into this refactor and will make consuming that JSON much easier and more consistent than if we had tried this prior to the refactor.

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Additionally, I've started printing out the error codes associated with composition errors, just like rover supergraph compose does, which fixes #508.

$ rover supergraph compose --config ../supergraph-demo/supergraph.yaml
error[E029]: Encountered 3 build errors while trying to build a supergraph.

Caused by:
    EXTERNAL_UNUSED: [inventory] Product.id -> is marked as @external but is not used by a @requires, @key, or @provides directive.
    KEY_FIELDS_SELECT_INVALID_TYPE: [products] Product -> A @key selects idddd, but Product.idddd could not be found
    KEY_NOT_SPECIFIED: [inventory] Product -> extends from products but specifies an invalid @key directive. Valid @key directives are specified by the originating type. Available @key directives for this type are:
        @key(fields: "idddd")
        @key(fields: "sku package")
        @key(fields: "sku variation{id}")
    
              The subgraph schemas you provided are incompatible with each other. See https://www.apollographql.com/docs/federation/errors/ for more information on resolving build errors.

$ rover subgraph check rover-supergraph-demo --schema ../supergraph-demo/subgraphs/products.graphql --name products
Checking the proposed schema for subgraph products against rover-supergraph-demo@test
error[E029]: Encountered 3 build errors while trying to build subgraph "products" into supergraph "rover-supergraph-demo@test".

Caused by:
    EXTERNAL_UNUSED: [inventory] Product.id -> is marked as @external but is not used by a @requires, @key, or @provides directive.
    KEY_FIELDS_SELECT_INVALID_TYPE: [products] Product -> A @key selects idddd, but Product.idddd could not be found
    KEY_NOT_SPECIFIED: [inventory] Product -> extends from products but specifies an invalid @key directive. Valid @key directives are specified by the originating type. Available @key directives for this type are:
        @key(fields: "idddd")
        @key(fields: "sku package")
        @key(fields: "sku variation{id}")
    
              The changes in the schema you proposed for subgraph products are incompatible with supergraph rover-supergraph-demo@test. See https://www.apollographql.com/docs/federation/errors/ for more information on resolving build errors.

[EverlastingBugstopper]

note to me: might want to try to do an e2e test on that GitContext constructor and make sure the proper env vars are being read

[lrlna]

What a feat! This is coming together nicely. Really really into the structure of rover-client now!