smith: generate a federation supergraph with @link

[lrlna]

A progressive list of federation directives apollo-smith should add and generate to create a supergraph:

@link

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1. @link directive on schem definition pointing to https://specs.apollo.dev/link/v1.0
2. import argument for @link https://specs.apollo.dev/link/v1.0/#@link.import
3. as argument for @link https://specs.apollo.dev/link/v1.0/#@link.as
4. url argument for @link https://specs.apollo.dev/link/v1.0/#@link.url
5. use arbitrary to generate other @link directive uses