Add bundle generate variant for dashboards (#1847)

## Changes

This change adds the `databricks bundle generate dashboard` command.

The command requires one of three flags:
* `--existing-id` to generate configuration for an existing dashboard by
its ID.
* `--existing-path` to generate configuration for an existing dashboard
by its path in the workspace file system.
* `--resource` to generate the `.lvdash.json` dashboard file for a
dashboard that's already defined in the bundle. This option does not
impact the YAML configuration.

A typical workflow could look like this:
1. Use the command with `--existing-id` or `--existing-path` for a
starting point
2. Run `bundle deploy` to deploy a copy of the dashboard
3. Run `bundle open` to open this copy in your browser
4. Navigate to the draft mode and make modifications
5. Run `bundle generate dashboard` with `--resource` to update the local
`.lvdash.json` file with the remote modifications

## Tests

* Unit tests.
* Manual walkthrough as documented in the [Dashboard for NYC Taxi Trip
Analysis
example](https://github.com/databricks/bundle-examples/tree/main/knowledge_base/dashboard_nyc_taxi).

bundle/config/generate/dashboard.go

@@ -0,0 +1,18 @@
+package generate
+
+import (
+	"github.com/databricks/cli/libs/dyn"
+	"github.com/databricks/databricks-sdk-go/service/dashboards"
+)
+
+func ConvertDashboardToValue(dashboard *dashboards.Dashboard, filePath string) (dyn.Value, error) {
+	// The majority of fields of the dashboard struct are read-only.
+	// We copy the relevant fields manually.
+	dv := map[string]dyn.Value{
+		"display_name": dyn.NewValue(dashboard.DisplayName, []dyn.Location{{Line: 1}}),
+		"warehouse_id": dyn.NewValue(dashboard.WarehouseId, []dyn.Location{{Line: 2}}),
+		"file_path":    dyn.NewValue(filePath, []dyn.Location{{Line: 3}}),
+	}
+
+	return dyn.V(dv), nil
+}

cmd/bundle/generate.go

@@ -16,6 +16,7 @@ func newGenerateCommand() *cobra.Command {
 
 	cmd.AddCommand(generate.NewGenerateJobCommand())
 	cmd.AddCommand(generate.NewGeneratePipelineCommand())
+	cmd.AddCommand(generate.NewGenerateDashboardCommand())
 	cmd.PersistentFlags().StringVar(&key, "key", "", `resource key to use for the generated configuration`)
 	return cmd
 }