Fix U2M OAuth flows in Azure

[mgyucht]

What changes are proposed in this pull request?

This PR fixes some issues affecting the end-to-end workflow using External Browser auth in Azure. In addition to the base PR by @renaudhartert-db:

1. This corrects the default redirect URL for the Databricks CLI to http://localhost:8020, matching the Go SDK.
2. This fixes OAuth token caching to include the actual scopes requested in the hash (otherwise, if no scopes are requested, that is treated as null today, causing an NPE).
3. This removes legacy Azure-specific handling for U2M Auth, where we requested the 2ff814a6-3304-4ab8-85cb-cd0e6f879c1d/user_impersonation permission. This is not needed anymore, since we login with the Databricks CLI App by default.
4. This fixes serialization of query parameters, using the standard UrlEncoder instead of the very approximate version replacing spaces with %20.
5. This changes the default scopes requested to be offline_access and all-apis, matching the defaults for the Go SDK & CLI.

How is this tested?

I wrote a demo app using U2M OAuth to authenticate and make an API request:

package com.databricks.sdk.demo;

import com.databricks.sdk.WorkspaceClient;
import com.databricks.sdk.core.DatabricksConfig;

public class App {
    public final static void main(String[] args) {
        DatabricksConfig config = new DatabricksConfig()
          .setHost("https://<azure workspace host>")
          .setAuthType("external-browser");
        WorkspaceClient client = new WorkspaceClient(config);

        System.out.println(client.currentUser().me());
    }
}

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