In Conda Store, hide filter and show installed

[gabalafou]

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This PR is a stepping stone to a minimum viable user experience for Conda Store: https://github.com/Quansight/conda-store/issues/228

Subsequent PRs will change the search box so that instead of merely filtering down a large list of packages locally, it will use search results returned from the server.

[github-actions[bot]]

:point_left: Launch a binder notebook on the branch Quansight/gator/conda-store-shows-installed

[fcollonval]

Thanks for trying the suggestion @gabalafou

Let's keep getInstalledPackages optional and use a boolean. Regarding the name for it, what about requiresSearch - the trouble with hasSearchProvider is that the local conda or mamba have a search capability (the endpoint is even available already).