Add a component for the metrics subcommand

[sgreenbury]

The svelte component will be able to:

1. Do the equivalent of calling the metrics subcommand with an input of a set of search fields (SearchParams) and returning SearchResults
2. Parse the resulting dataframe (e.g. as JSON). There is JsonWriter in polars that could be called in rust_backend to provide the result as a JSON string and then a frontend method can be added to rust_worker.ts
3. Display the parsed search results within the web app. For example this could be in a panel for displaying tables.

Both 2) and 3) are also tasks in #9 -- the logic to do this should probably be shared.

[sgreenbury]

Completed with #13 merged.