Allow customizing the sshConfigPath

In our environment, we strictly manage what the `$HOME/.ssh/config` file looks like - we replace it if it doesn't match our exact standard. In our config we allow custom configs in another directory (`$HOME/.ssh/config.d`). This conflicts with the Jetbrains Coder plugin because the plugin writes custom SSH configs into [`settings.sshConfigPath`](https://github.com/coder/jetbrains-coder/blob/39a29426e535208053d987562cc874a11925bc54/src/main/kotlin/com/coder/gateway/settings/CoderSettings.kt#L130) which is not configurable.

Please consider making the `settings.sshConfigPath` setting something that can be overridden. Personally we'd love to see it done as an environment variable... or we would need some programatic way to configure the path for the plugin via the shell.