Add map and pair helper functions for bundle templates (#665)

## Changes
Go text templates allows only specifying one input argument for
invocations of associated templates (ie `{{template ...}}`). This PR
introduces the map and pair functions which allow template authors to
work around this limitation by passing multiple arguments as key value
pairs in a map.

This PR is based on feedback from the mlops stacks migration where
otherwise a bunch of duplicate code is required for computed values and
fixtures.

## Tests
Unit test

libs/template/helpers.go

@@ -15,6 +15,11 @@ func (err ErrFail) Error() string {
 	return err.msg
 }
 
+type pair struct {
+	k string
+	v any
+}
+
 var helperFuncs = template.FuncMap{
 	"fail": func(format string, args ...any) (any, error) {
 		return nil, ErrFail{fmt.Sprintf(format, args...)}
@@ -27,4 +32,23 @@ var helperFuncs = template.FuncMap{
 	"regexp": func(expr string) (*regexp.Regexp, error) {
 		return regexp.Compile(expr)
 	},
+	// A key value pair. This is used with the map function to generate maps
+	// to use inside a template
+	"pair": func(k string, v any) pair {
+		return pair{k, v}
+	},
+	// map converts a list of pairs to a map object. This is useful to pass multiple
+	// objects to templates defined in the library directory. Go text template
+	// syntax for invoking a template only allows specifying a single argument,
+	// this function can be used to workaround that limitation.
+	//
+	// For example: {{template "my_template" (map (pair "foo" $arg1) (pair "bar" $arg2))}}
+	// $arg1 and $arg2 can be referred from inside "my_template" as ".foo" and ".bar"
+	"map": func(pairs ...pair) map[string]any {
+		result := make(map[string]any, 0)
+		for _, p := range pairs {
+			result[p.k] = p.v
+		}
+		return result
+	},
 }

libs/template/helpers_test.go

@@ -54,3 +54,18 @@ func TestTemplateUrlFunction(t *testing.T) {
 	assert.Len(t, r.files, 1)
 	assert.Equal(t, "https://www.databricks.com", string(r.files[0].(*inMemoryFile).content))
 }
+
+func TestTemplateMapPairFunction(t *testing.T) {
+	ctx := context.Background()
+	tmpDir := t.TempDir()
+
+	r, err := newRenderer(ctx, nil, "./testdata/map-pair/template", "./testdata/map-pair/library", tmpDir)
+
+	require.NoError(t, err)
+
+	err = r.walk()
+	assert.NoError(t, err)
+
+	assert.Len(t, r.files, 1)
+	assert.Equal(t, "false 123 hello 12.3", string(r.files[0].(*inMemoryFile).content))
+}

libs/template/testdata/map-pair/library/abc.tmpl

@@ -0,0 +1,3 @@
+{{- define "my_template" -}}
+{{- .foo}} {{.bar}} {{.abc}} {{.def -}}
+{{- end -}}

libs/template/testdata/map-pair/template/hello.tmpl

@@ -0,0 +1 @@
+{{template "my_template" (map (pair "foo" false) (pair "bar" 123) (pair "abc" "hello") (pair "def" 12.3)) -}}