Add client, rest, and export to known directives in apollo-utilities

[peggyrayzis]

This PR fixes an issue with apollo-link-state where adding the @client directive to the object key in #2710 broke cache.writeData:
apollographql/apollo-link-state#187

This fix is temporary until we can think of a more robust solution for directional directives like @client and @rest. With directional directives, the cache doesn't care about the directive since it only impacts the execution, not the results. This is in contrast to translational directives like @uppercase, where the result is altered.

[apollo-cla]

	Warnings
⚠️	There are library changes, but not tests. That's OK as long as you're refactoring existing code

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