ruff server: Support a custom TOML configuration file

[snowsignal]

Summary

Closes #10985.

The server now supports a custom TOML configuration file as a client setting. The setting must be an absolute path to a file. If the file is called pyproject.toml, the server will attempt to parse it as a pyproject file - otherwise, it will attempt to parse it as a ruff.toml file, even if the file has a name besides ruff.toml.

If an option is set in both the custom TOML configuration file and in the client settings directly, the latter will be used.

Test Plan

1. Create a ruff.toml file outside of the workspace you are testing. Set an option that is different from the one in the configuration for your test workspace.
2. Set the path to the configuration in NeoVim:

require('lspconfig').ruff.setup {
    init_options = {
      settings = {
        configuration = "absolute/path/to/your/configuration"
      }
    }
}

3. Confirm that the option in the configuration file is used, regardless of what the option is set to in the workspace configuration.
4. Add the same option, with a different value, to the NeoVim configuration directly. For example:

require('lspconfig').ruff.setup {
    init_options = {
      settings = {
        configuration = "absolute/path/to/your/configuration",
        lint = {
          select = []
        }
      }
    }
}

5. Confirm that the option set in client settings is used, regardless of the value in either the custom configuration file or in the workspace configuration.

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[GaetanLepage]

In the absence of a project configuration for ruff, will the LS load the seetings from ~/.config/ruff/ruff.toml like the CLI does ?

[charliermarsh]

It should yes -- not sure if it does yet though @snowsignal?

[snowsignal]

@GaetanLepage @charliermarsh I actually don't think we do this, yet - I've filed an issue to support this.

[charliermarsh]

I have one suggestion to simplify the implementation and semantics.