Automate the generation of bundle schema descriptions (#1007)

## Changes
This PR makes changes required to automatically update the bundle docs
during the CLI release process. We rely on `post_generate` scripts that
are executed after code generation with CWD as the CLI repo root.

The new `output-file` flag is introduced because stdout redirect does
not work here and would otherwise require changes to our release
automation CLI (deco CLI)

## Tests
Manually. Regenerated the CLI and the descriptions were indeed generated
for the CLI from the provided openapi spec.

.codegen.json

@@ -8,6 +8,9 @@
         ".codegen/cmds-account.go.tmpl": "cmd/account/cmd.go"
     },
     "toolchain": {
-        "required": ["go"]
+        "required": ["go"],
+        "post_generate": [
+            "go run main.go bundle schema --only-docs --output-file ./bundle/schema/docs/bundle_descriptions.json"
+        ]
     }
 }

cmd/bundle/schema.go

@@ -2,6 +2,7 @@ package bundle
 
 import (
 	"encoding/json"
+	"os"
 	"reflect"
 
 	"github.com/databricks/cli/bundle/config"
@@ -16,11 +17,18 @@ func newSchemaCommand() *cobra.Command {
 	}
 
 	var openapi string
+	var outputFile string
 	var onlyDocs bool
 	cmd.Flags().StringVar(&openapi, "openapi", "", "path to a databricks openapi spec")
 	cmd.Flags().BoolVar(&onlyDocs, "only-docs", false, "only generate descriptions for the schema")
+	cmd.Flags().StringVar(&outputFile, "output-file", "", "File path to write the schema to. If not specified, the schema will be written to stdout.")
 
 	cmd.RunE = func(cmd *cobra.Command, args []string) error {
+		// If no openapi spec is provided, try to use the environment variable.
+		// This environment variable is set during CLI code generation.
+		if openapi == "" {
+			openapi = os.Getenv("DATABRICKS_OPENAPI_SPEC")
+		}
 		docs, err := schema.BundleDocs(openapi)
 		if err != nil {
 			return err
@@ -39,6 +47,16 @@ func newSchemaCommand() *cobra.Command {
 				return err
 			}
 		}
+
+		// If outputFile is provided, write to that file.
+		if outputFile != "" {
+			f, err := os.OpenFile(outputFile, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
+			if err != nil {
+				return err
+			}
+			defer f.Close()
+			cmd.SetOut(f)
+		}
 		cmd.OutOrStdout().Write(result)
 		return nil
 	}