Connections RPC front end

[seancolsen]

In #3524, we added the following RPC APIs on the backend:

• connections.add_from_known_connection
• connections.add_from_scratch

This PR changes the front end to use those new RPC APIs.

In doing so, it also adds a new general-purpose utility, which I named json-rpc-client-builder. This code is Mathesar-agnostic, so I put it into a new place in our codebase — a packages directory. See the README.md files for more info.

The Mathesar-specific code for these new API methods is is src/api/rpc/ and you'll notice it's quite small. By following the pattern established here, we'll be able to implement the client side of new RPC methods with very little code.

I strongly recommend reviewing this PR commit-by-commit.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[pavish]

Looks good to me!