Refactor to new composition approach (#573)

This is part of the big composition refactor (kicking off rewrite) and
includes:

1. Remove `harmonizer`, we won't be publishing that Deno-based crate
anymore
2. Remove `supergraph`, those binaries are now built elsewhere
3. Add `apollo-composition`, the new crate for coordinating the various
pieces of composition. _Sort_ of a replacement for Harmonizer, but it
doesn't bring a JavaScript runtime (like Deno), instead relying on
consumers to implement an interface with a runtime of their choosing.
4. Updates to `apollo-federation-types` to facilitate new consumers of
`apollo-composition`
5. Simplify CI, since we're doing a lot less in this repo now

<!-- [CNN-421] -->

[CNN-421]:
https://apollographql.atlassian.net/browse/CNN-421?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

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Co-authored-by: Lenny Burdette <[email protected]>
Co-authored-by: Trevor Scheer <[email protected]>
Co-authored-by: Taylor Jones <[email protected]>
Co-authored-by: Jeffrey Burt <[email protected]>
Co-authored-by: Nicholas Cioli <[email protected]>