Alias variables block in the Target struct (#1748)

## Changes
This PR aliases and overrides the schema associated with the variables
block in `target` to allow for directly specifying a variable value in
the JSON schema (without an levels of nesting). This is needed because
this direct value is resolved by dynamically parsing the configuration
tree.

https://github.com/databricks/cli/blob/ca6332a5a4325aff1be848536f45d13bd74d93b3/bundle/config/root.go#L424

## Tests
Existing unit tests.

bundle/config/root_test.go

@@ -139,7 +139,7 @@ func TestRootMergeTargetOverridesWithVariables(t *testing.T) {
 		},
 		Targets: map[string]*Target{
 			"development": {
-				Variables: map[string]*variable.Variable{
+				Variables: map[string]*variable.TargetVariable{
 					"foo": {
 						Default:     "bar",
 						Description: "wrong",

bundle/config/target.go

@@ -38,7 +38,26 @@ type Target struct {
 	// Override default values or lookup name for defined variables
 	// Does not permit defining new variables or redefining existing ones
 	// in the scope of an target
-	Variables map[string]*variable.Variable `json:"variables,omitempty"`
+	//
+	// There are two valid ways to define a variable override in a target:
+	// 1. Direct value override. We normalize this to the variable.Variable
+	//    struct format when loading the configuration YAML:
+	//
+	//   variables:
+	//     foo: "value"
+	//
+	// 2. Override matching the variable.Variable struct.
+	//
+	//   variables:
+	//     foo:
+	//       default: "value"
+	//
+	// OR
+	//
+	//   variables:
+	//     foo:
+	//       lookup: "resource_name"
+	Variables map[string]*variable.TargetVariable `json:"variables,omitempty"`
 
 	Git Git `json:"git,omitempty"`
 

bundle/config/variable/variable.go

@@ -16,6 +16,11 @@ const (
 	VariableTypeComplex VariableType = "complex"
 )
 
+// We alias it here to override the JSON schema associated with a variable value
+// in a target override. This is because we allow for directly specifying the value
+// in addition to the variable.Variable struct format in a target override.
+type TargetVariable Variable
+
 // An input variable for the bundle config
 type Variable struct {
 	// A type of the variable. This is used to validate the value of the variable

bundle/internal/schema/main.go

@@ -21,6 +21,22 @@ func addInterpolationPatterns(typ reflect.Type, s jsonschema.Schema) jsonschema.
 		return s
 	}
 
+	// The variables block in a target override allows for directly specifying
+	// the value of the variable.
+	if typ == reflect.TypeOf(variable.TargetVariable{}) {
+		return jsonschema.Schema{
+			AnyOf: []jsonschema.Schema{
+				// We keep the original schema so that autocomplete suggestions
+				// continue to work.
+				s,
+				// All values are valid for a variable value, be it primitive types
+				// like string/bool or complex ones like objects/arrays. Thus we override
+				// the schema to allow all valid JSON values.
+				{},
+			},
+		}
+	}
+
 	switch s.Type {
 	case jsonschema.ArrayType, jsonschema.ObjectType:
 		// arrays and objects can have complex variable values specified.