feat: adds rover install --plugin supergraph-{0,2}

[EverlastingBugstopper]

fixes #1046.

this PR adds rover install --plugin supergraph-{0,2} and removes rover-fed2.

the curl and npm installers will both run rover install --plugin supergraph@latest-0 and rover install --plugin supergraph@latest-2. you can run rover install --plugin [email protected] to get an exact version.

rover supergraph compose will check if a new supergraph binary has been released and download that to use it for composition if it is available. these network checks can be skipped with --skip-update in order to use the plugins that were installed at the same time as rover. you can also specify federation_version: =2.0.0 in supergraph.yaml to get an exact version that will only download the plugin once and won't check for updates unless the number changes.

the npm installer now puts rover in ~/node_modules/.bin instead of ~/node_modules/binary-install/bin, and sets APOLLO_NODE_MODULES_BIN_DIR so that rover knows where it lives and can install the supergraph binaries there as well.

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