Add override to support YAML inputs for apps (#921)

## Changes
<!-- Summary of your changes that are easy to understand -->

Take @andrefurlan-db 's original
[commit](https://github.com/databricks/cli/compare/databricks:6e21ced...andrefurlan-db:12ed10c)
to add `apps` support to the CLI and add the yaml file-support as an
override (the apps routes are already apart of the Go SDK and are
available for use in the CLI)

**NOTE: this feature is still private preview. CLI usage will be
internal only**

## Tests
<!-- How is this tested? -->

cmd/workspace/apps/overrides.go

@@ -0,0 +1,58 @@
+package apps
+
+import (
+	"fmt"
+
+	"github.com/databricks/cli/cmd/root"
+	"github.com/databricks/cli/libs/cmdio"
+	"github.com/databricks/cli/libs/flags"
+	"github.com/databricks/databricks-sdk-go/service/serving"
+	"github.com/spf13/cobra"
+)
+
+func createOverride(cmd *cobra.Command, deployReq *serving.DeployAppRequest) {
+	var manifestYaml flags.YamlFlag
+	var resourcesYaml flags.YamlFlag
+	createJson := cmd.Flag("json").Value.(*flags.JsonFlag)
+
+	// TODO: short flags
+	cmd.Flags().Var(&manifestYaml, "manifest", `either inline YAML string or @path/to/manifest.yaml`)
+	cmd.Flags().Var(&resourcesYaml, "resources", `either inline YAML string or @path/to/resources.yaml`)
+
+	cmd.Annotations = make(map[string]string)
+
+	cmd.PreRunE = root.MustWorkspaceClient
+	cmd.RunE = func(cmd *cobra.Command, args []string) (err error) {
+		ctx := cmd.Context()
+		w := root.WorkspaceClient(ctx)
+		if cmd.Flags().Changed("json") {
+			err = createJson.Unmarshal(&deployReq)
+			if err != nil {
+				return err
+			}
+		} else if cmd.Flags().Changed("manifest") {
+			err = manifestYaml.Unmarshal(&deployReq.Manifest)
+			if err != nil {
+				return err
+			}
+			if cmd.Flags().Changed("resources") {
+				err = resourcesYaml.Unmarshal(&deployReq.Resources)
+				if err != nil {
+					return err
+				}
+			}
+		} else {
+			return fmt.Errorf("please provide command input in YAML format by specifying the --manifest flag or provide a json payload using the --json flag")
+		}
+		response, err := w.Apps.Create(ctx, *deployReq)
+		if err != nil {
+			return err
+		}
+
+		return cmdio.Render(ctx, response)
+	}
+}
+
+func init() {
+	createOverrides = append(createOverrides, createOverride)
+}

libs/flags/yaml_flag.go

@@ -0,0 +1,42 @@
+package flags
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/ghodss/yaml"
+)
+
+type YamlFlag struct {
+	raw []byte
+}
+
+func (y *YamlFlag) String() string {
+	return fmt.Sprintf("YAML (%d bytes)", len(y.raw))
+}
+
+// TODO: Command.MarkFlagFilename()
+func (y *YamlFlag) Set(v string) error {
+	// Load request from file if it starts with '@' (like curl).
+	if v[0] != '@' {
+		y.raw = []byte(v)
+		return nil
+	}
+	buf, err := os.ReadFile(v[1:])
+	if err != nil {
+		return fmt.Errorf("read %s: %w", v, err)
+	}
+	y.raw = buf
+	return nil
+}
+
+func (y *YamlFlag) Unmarshal(v any) error {
+	if y.raw == nil {
+		return nil
+	}
+	return yaml.Unmarshal(y.raw, v)
+}
+
+func (y *YamlFlag) Type() string {
+	return "YAML"
+}